Files play in Apple TV but not iTunes

So I am in the process of converting my digital library from .mkv to .mp4 and I'm running into a problem that I can't understand. While all of my converted files work properly on the ATV, several of them simply refuse to play in iTunes. The files load but then get stuck at the first frame of the video. I can't seem to figure this out. Has anyone had this problem before?

Interesting. I'm gonna have to research this. Thanks for the tip.