X box 360 wont play some movies from usb flash drive?

i am working on putting all my dvd's onto my computer and then onto a portable hard drive that i can play on my x box. most of my movies will play fine but i get a movie once in a while that just wont show up on my x box. sometimes i have to encode it with a different program because of problems with my other encoder but all the specs look the same and it plays fine on the computer. i like to use video - xvid mpeg-4 and either 24 or 30 fps and the audio is usually ac-3 or aac