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.
Try turning on your external HD before you boot the OS. If your drive does not have a power switch, unplug it and plug it back in again. Wait a few seconds for the HD to fully start up and then power up your computer. Once the OS is booted up, make sure you see the drive mounted in your file system and then start itunes. There is a good chance that this will fix your problem.
I had this problem when one song was selected. During playback artwork was shown only for one song. I fixed it by marking all songs and pasting artwork in properties.
Kijanki: Are you referring to an iTunes properties location? Not sure what you are referring to there.