When I first downloaded '11' I had a few album covers missing. Not many, maybe ten or 15 of about 450 albums. I went to each album and clicked the first song and then checked the details (cmd-I), nothing. But when I checked the next song it was there. So I just copied it to the missing one, checked the rest of the songs on the album to be sure, and it was okay after that.
It was a bit tedious, but like I say there weren't that many so it was worth it to me to check them out. Funny thing is that in the last iTunes I had one album that would never show the artwork for some reason, just that one. The pics were there when I opened the item detail on each song, but it never showed in the main library section. After downloading '11', that album cover now shows up every time. Go figure...
