When I download a movie from iTunes the cover art disappears. I try get info and add the art, it doesn't show up. So it's just gray. It opens and plays fine. Apple can't figure it out. It's an nas drive. When I use a firewire drive I get the art

When I download movies from iTunes to my NAS drive, I don't get the cover art. It shows up while its downloading, but when it's finish it disappears. If it has an iTunes extra the cover art is there. I tried get info and add the cover art, it never shows. Regardless of how I add it. Funny thing is, if I download the same movie to my external firewire drive, no problem its there. I've called Apple with no success. I've even tried copying the movie with the art from the firewire drive to replace the file on then NAS drive I still loose the movie cover art. I also tried several times to re download it, no luck. Any ideas? I'm out of ideas.