iTunes and NAS

I keep my iTunes music library on a LaCie Network SpaceMAX.

I frequently purchase predominently film and TV (rather than music) through iTunes. I have almost 3TB of film and TV, some of which was pre-existing, but about half of which has been bought over time from iTunes, over various accounts and because of the shear volume, I keep these files on a NAS.

For about two years, all video and TV files appear to download incorrectly or as corrupted files. Though the file downloads, titled and with it's relevent album information (and in the relevent secion on my NAS) the files are unplayable through my computer:

"This movie requires QuickTime, which is not supported by this version of iTunes."

The workaround appears to be to delete and re-add the files but this is unacceptable, for all but the odd file, and it is only film and TV files. Music and podcasts do not have this problem. The files, unless manually removed and re-added do not play on iPhones, iPods or iPads either. They do play on the Apple TV but appear as the same incomplete file (without cover art).

I know other people have had this issue but have yet to come to a satisfactory conclusion.

Is it a permissions issue? Does anyone anywhere have any solution to this? I am an extremely heavy user and to delete and re-add all 30+ new episodes, individually, as they download each week is beyond a chore.