A difficult aspect of the jhana practice for me is that not all the jhanas are clear. I wonder if anyone here has a similar experience. My new-year's retreat will be focused on the jhanic arc, and trying to get all the jhanas extra extra clear. I would welcome any and all tips, comments, etc.
The clearest jhanas for me are 1, 2, 3, 5 and 7. Here is a rundown of me running through the jhanic arc:

• Jhana 1, tight focus, clear downward-cone eye posture, a pleasure with rushed/orgasmic quality which I take to be piti. The pleasure is moderate, and I can modulate the pleasure to make it more intense, but the mind gets very agitated.
• Jhana 2, clear straight-ahead eye posture, the rushed quality of pleasure is more toned down, happiness is also there, some kind of nimita can be made to arise here, though never very stable.
• Jhana 3, rushed quality of pleasure is gone, just happy soft joyful tingly pleasure, eye posture places emphasis on the edges of the visual field, more tranquil.
• Jhana 4, is this place which comes after jhana 3 and before jhana 5. It isn't very clear. I feel the tendency to raise my eyes a little bit maybe, but they don’t stay there. Pleasure dies down, for sure, it’s nice to be there but no bliss. The clearest sign among the classic signs is that there is a whitening of the visual/spacial field. It doesn’t feel super stable, unlike what I’ve been told. I would even doubt it’s really a full-fledged 4th, except that it is followed by a clear shift which has to be the 5th.
• Jhana 5. Suddenly there is a ball of space inside my head. This ball expands, sometimes but not consistently, and sometimes gets quite big, but even when it isn’t big it is unmistakably different from any other state. Body sensations feel like a distraction. I can’t quite figure out what to do to make it expand. If I intensely intend for it to expand it sometimes does, and this is accompanied by a rushed pleasure which is reminiscent of 1st-jhana but not exactly the same.
• Jhana 6. Eventually the ball of space seems to shift out-of-phase, and I’m somewhere else, which I take to be 6th jhana. I do notice here there is a sensation which I once have taken to be “my self”, whatever that means, but which is nowadays clearly just a sensation. Here I usually focus on that, and on an adjacent sensation which I could call “knowing”, or “knowing where I am”, or something like that. As a sensation it is quite reassuring as if “I am not lost”, but I doubt that in reality I am any more or less lost when the sensation is present, it’s just an association I think. Assuming I’m focusing on vaguely the right thing, which I’m really not sure about.
• Jhana 7. However, intending to “drop” this sensation of knowing, I come to the next place. There is a place behind the middle of the forehead, just at the top of the nose, such that when the focus is placed there it actually feels as if the focus is out of sync with everything. It’s a really weird feeling. I’m taking that to be Jhana 7, without much certainty. It is unstable for me, I have to stay very carefully present to sustain it, and I always shift in and out of it, so I’m in that place maybe 50-80% of the time. As it matures, it starts to feel as if this out-of-syncness is its own “thing”, and that I am on some level recognizing it from a certain vantage point “*that* is the out-of-syncness”. I am not really certain of this interpretation.
• Jhana 8. But what I do next is to drop the recognition/vantage point. Not just with respect to the nothingness/out-of-syncness of the 7th jhana, but just drop it entirely. And the result is very very trippy. It’s like I cant really latch on to anything, because that would require re-establishing the vantage point. But it’s as if the mind tries to re-establish it, again and again, and yet doesn’t allow itself to do it, because it remembers (somehow) to not establish it, again and again. This yes-but-no state is also quite unstable for me, I find myself accidentally restoring the vantage point, usually on some wide subtle background sensation, and sometimes I can’t get back to 8th without going through 7. Sometimes I need to go way back down to get back in there. Usually 5 is enough, but I’ve had to go as low as 3. It feels as though its a bit part of the nature of the state that I feel a bit lost, it’s hard to make decisions, maybe because they require the vantage point, or maybe because they don’t feel like decisions without it.
• Fruition. Anyway, some of the times when I get to 8, my mind is calm enough that it doesn’t get drawn away from the yes-but-no. Jhana 8 “stabilizes” of sorts, though it’s more like I get used to the rhythm. And then I notice that there is still something there, like a very subtle “lingering consciousness”. Just gently noticing it without trying to add anything to it, seems to cause it to be let go. There is a sense of something breaking through to another side, an electrical circuit being closed on the ajna chakra, and there is a discharge, which is usually quite pleasant. It is sometimes felt as discharging through the ajna and reappearing at the heart chakra for some reason. But sometimes I only notice the discharge on the ajna.
  Coming out of it there has been a significant increase in clarity, and I no longer appear to be in any jhana anymore.
• Or: Jhana 9 (no kidding, here’s a tour of all 13 jhanas by Kenneth Folk and Nick Haley). I can also, instead of focusing on letting go, attempt to push through to the ajna chakra. It goes through, and then up the forehead. Suddenly the ajna chakra is pulsating, blissfull, etc. This shift is very obvious and very clear, but after that it is all murky.
  I got the remaining 4 jhanas on retreat once, and they seemed reasonably clear then (not super clear, just ok), but off retreat it’s all murky territory, and definitely the 13th I only had that one time on retreat.

Hello Bruno, some time back I had done a series of 4 talks on discord with a few of my friends. These 4 talks are regarding the jhanas, how to learn them, what they feel like, etc

Talk 1 - Prerequisites
Talk 2 - Access concentration and the spectrum of its depth
Talk 3 - the first 4 jhanas
Talk 4 - the last 4 jhanas

In talks 3 and 4 between me and a few friends we had covered a lot of ground regarding how to learn these jhanas and what each feels like. 

Please see if you would like to check out these talks / discussions. Link:

​​​​​​​https://drive.google.com/drive/folders/1rtLrOyfiHzq_Ed0Go2B_zqxExa-Q49IJ?usp=drive_link

I had a very similar issue. I learned the jhanas from Right Concentration but three and above were not clear. In particular, I had trouble getting a solid, stable 4th. 

I worked with Nick Grabovac, who was actually a teacher trainer working under Culadasa before Culadasa had his troubles, so he was originally a TMI guy, but he worked with me in a Right Concentration framework. I spent maybe six months or so working with him (it's in my logs, if you are curious). To boil it down to a simple formula, Nick told me to just get really, really good at access centration and each jhana before moving on to the next one. I worked until I was getting insanely powerful first and seconds (electricity, quasi-orgasmic, all that stuff, which I was getting before, but now they were off the charts) and kept working them further, with the idea of really permeating the entire body so that there was no part of the body that was not permeated. I got my firsts to be bright white with all ambient sounds cut off. Then I started doing up-and-down drills, going from first to fourth and back again repeatedly, like a trampoline, until I was getting equanimity so thick you could cut it with a knife (Nick's terms) in fourth. I have never got a stable 8th, and I have never tried for anything beyond that, but I got full absorption in 1 to 7. So, in short, this technique, Nick's technique, is to get really good at each one, to work at it like a craftsman, or a musician with the highest standards, and just put the time in, again, and again, and again, and never say "good enough" until you get to the stage where you cannot say "good enough" because the absorption is so complete that no verbal thought or evaluative thought can form. 

I personally wouldn't stress about this (although, I might give that response to every question... ) . I've found that over the course of the last couple of years my jhanas have changed pretty significantly structurally depending on where I am in my practice. So around my "holy shit" moment, my jhanas were intensely strong, lasted for hours or days, very clear, very "mystical". As practice progressed they got deeper and stranger (full body energies, seeing frame rates of reality, etc), but then as it continued they kinda mellowed out significantly, lost their importance, and even took on a bit of a disassociative quality (hell a lot of times things would be so peaceful and everything so soft that it wasn't until I looked back on a meditation and realized the jhanic factors were present). And that takes us to .... this, right here right Now, where I need to be doing my own investigations.

All that being said, I don't want to discourage you from going deeper into Jhanas. If this is a spot of your practice that rings true right now, go for it. I'm just trying to raise the point that sometimes struggling getting into jhanas, or the clarity of the factors, or the intensity of the whole thing etc is also important to investigate. There is a lot of context around the whole thing that is important too!

Yes I have experienced this. More concentration makes them clearer / more distinct.

You should consider if it is the best way to spend your time. The Buddha taught his that samatha and vipassana are two qualities of mind that should both be cultivated. So someone might decide how much effort, more or less, to put into jhanas in order keep samatha in balance with vipassana.