Several users have experienced an issue in which the iPad cannot play back certain movies purchased from the iTunes Store, despite the fact that said movies play properly on Macs and other devices linked to the same iTunes account.
Fixes
Convert to iPad format. Select the movie in iTunes on your computer, then go to the “Advanced” menu and select “Create iPad or Apple TV Version,” then re-sync your iPad with your computer and check for proper playback.
Turn off surround sound. Play the movie in iTunes on your computer in full-screen mode, then move the cursor over the small icon that resembles a bubble, and select the “stereo” option instead of “surround.” Re-sync your iPad then try again to play the movie.