I know with music I go through regular slumps when my ear improves and I hear in more detail. This increases my expectations and I forget how far I've come. Maybe you are seeing more, and clearer and so you expect to render it in fuller detail, more fluidly but you haven't actually had a chance to incorporate that clearer vision into what you're drawing.
Time is a factor. I try to reserve judgement and just put in each day's work and then evaluate after three months have passed. I always see some progress if I give myself enough time to do the slow development work.
Maybe your expectations of fast improvement are putting on some pressure and taking away some of the fun of drawing? You can't replace days and weeks with hours. You need time away from drawing to let your mind and body develop. Sometimes just drawing for an hour a day for a month can do much more than trying to draw for six hours a day for a week.
