quicktime won't play a .mp4 file. Why not?

I am trying to play a video file in quicktime and get the error message 'The movie is not in a format that QuickTime Player understands." It is a .mp4 file, which I thought quicktime should play. Also tried VLC media player, but not working either. This is a file burned from a DVD, a TV box set. The other files so far fromt the same DVD have played no problem. Just this fifth episode, won't play. Any ideas?