A lot of Kaplan materials are really hard. When you take their full-lengths, they make up for the hardness by giving you a nice curve, but this isn't comforting when all you see is a percentage.
It sounds like you've been very thorough in your content review. Your disappointing diagnostic and practice material performance is probably due to the fact that you're not used to MCAT style questions or MCAT pacing. My advice would be to take a few full-length practice tests right away and see where you are. You will probably be pleasantly surprised.