Album artwork shows up in iTunes, but not on iPod?

I got a new iPod nano on Christmas, downloaded all my music and playlists onto it, and everything went fine. The next day, I updated to the new version of iTunes (10.5.2), got some new songs and synced my iPod again. This time when I was scrolling through my music I noticed that most of the album artwork had disappeared except for the music I had just bought (plus a few random others). Like 90% of it is all gone, but it ALL shows up on iTunes on my computer.

I already tried "get album artwork", and I can't find any settings that will fix it. Is this just a glitch in this version of iTunes or am I missing something? Anyone know how to fix it? It's really annoying. 😕