Excellent, if you can access this at the drop of a hat then you've got some good concentration skills.
I did try some few times to take them as objects, but usually they stop coming if I don't focus on the breath. Well, I guess I have to keep trying and figure it out.
What you want to avoid doing is
trying to make them increase or, if you're doing straight concentration practice at least, investigating them any further; just observe, maintain a gentle focus on the sensations themselves and stay with them as consistently as possible. They become stronger of their own accord if you just keep the focus on them, but it's not a problem if you find yourself going back to the breath as your main object. It's a subtle balance that's required but I see no reason why you wouldn't be able to hit a firm 1st jhana with your concentration skills as they are, just experiment with the way the mind seems to cradle those sensations, how it takes them as an object and relax into it.
Part of the reason you may be finding it difficult to take the sensations as an object could be the way you've been practicing before, these sensation are, at first anyway, more subtle and less obvious than the breath so you might find that returning to the breath is just a habitual thing. Play around with it and see what works best for you, but always with the intent to focus on that object, whatever it may be, as consistently as possible and without struggling.