iTunes dropping album artwork


Does anybody know why iTunes continues to drop album artwork? the artwork shows up in the "Now Playing" Box but not in the main menu of albums. It is a constant pain to keep re-stocking album covers.

Any advice is appreciated.
puerto
Are you using an external HD? Also, what format are you using for your music files?
All of my files are in aiff. I get the album artwork by click and drag from the internet. Yes I use an external HD.