Album Artwork continually being deleted

For whatever reasons, iTunes keeps deleting artwork. All the time.

It doesn't matter whether the file is AAC or MP3, downloaded or imported, old or new. It just keeps removing the album art.

And when you get it back, it only last a while before being removed again.

And even if it does come back on iTunes - the ones with album art on iTunes, don't have album art on my iPhone!

I've tried wiping everything off my iPhone and updating iTunes...but that's made the problem even worse.

Any ideas?