This problem is most commonly caused by iTunes not being able to play the audio in a movie. You will see the first frame of the movie in iTunes, but it won't play.
iTunes won't be able to play a video containing an unsupported audio type. The most common audio type that causes this is AC3 (Dolby Digital audio). A video with an AC3 audio will play on an AppleTV, but not on a Mac running iTunes.
To fix this, make sure you don't have the 'AC3 Passthrough' option set in Advanced Preferences. Set it to 'Passthrough & Convert', 'Convert to AAC' or 'Converter to AAC (surround)'.
If you find changing this setting doesn't fix the playback, contact support and let us know about the problem. Include your log file in your message as well.