iTunes artwork will not allow manual change.

In iTunes 10.7 (21) I am unable to change artwork on my music. Not only will Clear Downloaded Artwork not work anymore, but when I either drag and drop the correct artwork to the album or go through Get Info and the Artwork and Add, it still fails to do what I ask.

Does anyone know why this would be happening? Is it a known issue? I've searched and found people having trouble with iTunes Match not allowing changes, but I'm not using that service.

Thanks

Tony