Why dont all avi files work??

Hi all. Can someone help me?? I started downloading movies from here to put on my flash drive, so I could plug into my ps3, blu ray player, or my daughters new tv. Most of the films I've downloaded have been avi files, with the exception of one or two mp4 files.

The thing is, some of the avi files play fine on playstation and daughters tv. But the rest don't play at all and show an "unsupported file" or "unsupported format" message.

I thought one avi file was the same as any other, and if one played, then they all would play.

Can someone please tell me if there's anything I can do??

Thanks in advance.