Something odd, explanation?

I put album covers on all of my songs. When I'm in iTunes, at least half of my songs say that there is no artwork available, even though I put the covers on. However, on my iPod, the cover is there. The cover is obviously there because I can see it on my iPod so how come on iTunes for some songs it says there's no artwork? I suppose it really doesnt matter as long as when I play a song I see the cover, I'm just curious as to why this happens.