MP4 movie does play in QuickTime but not in iTunes

MP4 movie does play in QuickTime but not in iTunes

I have one MP4 movie that plays just fine in QuickTime (I've run it from
beginning to end without problem).
However, when I imported it into iTunes, it can load but cannot play.
Clicking on the play/pause button does not work. Nevertheless, I can
advance the progress bar to any point in the movie, but the screen just
shows a still image of that point.
This is really strange since as far as I know, iTunes plays movies with QT.
So, what is the possible cause of this problem and how to fix it? Thanks
in advance.



My environmental specs: OS X 10.8.4, iTunes 11.0.4 (4), QuickTime Player
10.2 (603.17).