Based on the discussion and what I've heard from other Mac Geek Gab listeners who have this problem, it seems like iTunes has some sort of a cache that — in some cases — gets stuck (as in things go in but don't immediately come out!).
With that, it's reasonable to posit that clearing iTunes' cache would provide a solution. To do so:
1) Quit iTunes (and all of your other apps since you'll be restarting)
2) Go to your [home]/Library/Caches folder (simplest way to get to [home]/Library is to hold down Option and choose "Library" from the Finder's "Go" menu)
3) delete the com.apple.iTunes folder
4) Restart your Mac (probably not necessary, but better safe)
5) Launch iTunes and try again.
Curious to see if that helps. I don't have the problem here so am unable to test (but will file this away in case I ever need to try it! 😉).
-Dave