Movies in the "Video" folder not playing

When I just recently heard that PSP could supposedly play MP4 format videos just by copying them to the "Video" folder on the memstick, I was really excited. However, for some reason any video file I put in there shows up on the PSP's video menu as "unsupported data." Just to make sure it wasn't a problem with the files themselves, I copied a video that already worked from the MP folder into the Video folder. Still same results. Does anyone know what the problem here might be? Thanks in advance.

It's not supposedly, it does play mp4 vids
my guess, if it shows unsupported data means that they're not converted properly or not converted at all

there are a lot of programs to convert videos into mp4 psp format, some are freeware and work ok
There is one hanging around these forums called Xvid4PSP 4113

I use ImTOO MPEG encoder, it works okay (I use this one because xvid4psp requires win XP)