Specify which pieces you are working on, maybe you are approaching them wrong.
Well, one of the first "real" pieces that I learned was the C Major Prelude from the Well-Tempered Klavier, Book 1. At the time, I thought I could play it well, and I put it aside when I moved onto something else. When I came back to it a few months later, though, I couldn't play the arpeggiations evenly, and I felt that I couldn't achieve the same level of rhythmic consistency throughout the piece. I have since relearned this piece, however, and can play it much better now.
Where it is happening right now (and this is a good year now since I learned the Bach) is with the first movement of Mozart's A Major Sonata (K331), the "Tema con variazioni". I moved through the piece variation-by-variation, and I thought I had mastered each one before moving to the next, but now, after spending a good deal of time on the last variation, I'm finding that some of the earlier ones (like the second one, the one with all the triplets) aren't anywhere near mastered. I can't hit the trills in the second variation with the same level of precision anymore, and some of the triplets are rhythmically inaccurate.
These are just two examples, but this is a general pattern with me; I step away from a piece of a fragment of a piece for a while, and when I return, I find myself worse at it than when I left.
Thanks for your feedback, though, and you're probably right; I most likely am not mastering these pieces in their entireties before moving on, and it is biting me when I come back to them.