Some days (or most really) I have problems finding the proper ending. Sometimes I know/hear where I want to go, but I don't play it right. It's a lack of ear training quite possibly. Or "translation" experience.
And this is a problem even with my non-improv stuff, because I find that if I don't get the chord right by maybe the second or third try, what I heard in my head gets corroded by all of the "bad" stuff I played and then the idea is gone.