iTunes album artwork not showing

I operate a college internet-only radio station. The system we use is based on iTunes.

Each week, music distributors send me links to download new music. On average, I load about 20 albums per week into the system. The files are usually .zip files, and contain a .jpg for album artwork, a .pdf with band info, and a folder containing the .mp3 files for the songs. I unarchive the .zip file, then click-and-drag the resulting folder into iTunes.

About 1/3 of the time, everything works as expected. The music is copied into the iTunes library and the artwork appears as well. The majority of the time, though, the artwork does not appear in iTunes.

I've scrutinized these files, and am unable to spot any difference between the ones that work and the ones that don't. The heirarchy of the unarchived files and folders is the same, the images are roughly the same size.

I know how to go back in and add the artwork, but doing so for 15 albums each week is extra work I'd rather avoid. Any ideas why it works so inconsistently, and how to rectify that?

mahalo!

Tom G