howdy,
it is often assumed, erroneously, that meditaion is a linear growth curve from distracted life to nibbana. it aint. we're all a little different but a little the same too. so while the general trend eventually move in the above mentioned direction we are in a constant fluid process of overcoming our weaknesses and building on our strengths, taking them on as they arbitrarily arise and pass away.
as long as you keep practicing the long term trend will win out. a 'good' sit doesn't mean necessarily perfect concentration or blissful states, although these are not to be scoffed at. having an uncomfortable sit with low concentration is probably more valuable than a comfortable one if you stick with that and understand it.
what hindrances are coming up? are you agitated? restless? bored? tired or listless are you doubting yourself or the technique? i ask this because the hindrances are points which i used to avoid BECAUSE they are generally uncomfortable but are in fact the keys to the kingdom. once understood, or even just seen, one can move past them and settle down to real concentration.
