Why do some videos erase after syncing

[snapdragon]

After syncing, some videos (not rented) will erase from the iPad2 but some won't.

All will remain in the PC library and playlist, but the ones that were erased will not re-sync to the iPad.

Used 'manual manage' but that erases the music!

Also, tried drag and drop...doesn't work.

Any info?
Thanks!