Video wont play on my sidekick?!?

I have a sidekick LX 2008, and I downloaded 2 videos, they were MP3 so I converted them into MP4 so they will work on the sidekick to watch a video and it worked. So the 3rd video I downloaded and I converted it to MP4, but whenever I press play on the sidekick it says "The encoding or format of this file is not supported by the device" I dont get it! what do I have to do in order to make it work? It works fine on the computer, I can watch it there automatically. Please help! Thanks :D