Quicktime X is a great little piece of software. However, Quicktime X is very finicky on which codec/file format it can decode/play.
Luckily, there are DOZENS of alternative video/media players and extensions available for the Mac. Some of them are even FREE. Some of the better FREE ones include...
PERIAN (quicktime extension)
VLC Media Player
... And if none of those work, there is a possibility that, that video file may be corrupted.