Are they in your "My Videos" folder? That's where mine are and all are playing fine.
Yeah. I have it set to that folder. I have a lot of AVI and such from bittorrent and I didn't really expect those to play w/o come conversion of codec. I did use WinAVi and once again it produced a perfect WMV file and it looks great on the 360.
So to sum up my experience with the 360 video playback. After the d/l there were 5 movies in the folder on dashboard. I have about 100 movies in that folder, so it was weird that only five would show up. some of those files are AVI some are WMV, but the majority of WMV wouldn't work. I did as mickley stated and used Movie Maker to convert and that movie that didn't show up before now does. I wanted to try WinAVI to see how that would work and it works even better than movie maker. So to stream my movies I have to spend 20-30 minutes to convert them.