I've had the same problem so I tried just waiting to see if the problem was updating in Racks & Tags. After 24 hours, the art still didn't appear. Then I exported the picture from ODB, opened it in Photoshop, did a "save as" and reloaded it into ODB. Then I republished it to Racks & Tags and the art appeared almost instantly.
If I had to guess, I would say the problem is somewhere in the auto import of the cover art from Amazon (or wherever). All the problems I've had seem to be fixed by the above technique but maybe that's just coincidence