Could be a few reasons. First thing that popped into my head was that you said you did this by "right clicking on
a song" and did the get info thing. Problem with that might be that the album art is still associated with all the other songs in that album. One thing that might help you understand what I mean is that you can assign different album art to individual songs in the same album, particularly if you have crazy things like different capitalizations going on in your album titles. Make sure they are all the same, spelling, caps, punctuation. If you want to delete the art, select ALL songs in that album and then do the right click>get info thing.
Personally, I prefer to use a separate program to do any kind of album art, title, track, info editing. Some changes made in iTunes do not stay with the mp3 folders. I once wasted a few hours updating this kind of info. When I installed iTunes on my new laptop and added the same music files to the newly created library, none of the artwork was there. And that was one thing that I was sure to get right.
What you want is an ID3 tag editor. There are plenty out there, I prefer "tagscanner" because it is the first one I used and just learned it. It will rename files and folders, edit all values such as title, track number, artist, and album art. A little time consuming, but worth it because it sticks. You will have to do some research to get proper album art and dates and stuff. Wikipedia is a great source. I used images.google.com to find album art.
I would recommend doing this editing one artist at a time. When you are done with each artist, go into iTunes and delete that artist from your library. Then re-add the newly edited files to iTunes. I did about 60GB in a few days, working just about 30 minutes a day. Now I just edit stuff as I get it. Super fast. Library is much easier to use when you have all complete and accurate data.
