siddys: They must be in a specifically formatted MP4-SP or AVC codec to work.
Resolution: 320x240 or 368x208
Framerate: 29.97 (NTSC)
Audio: 48000 Hz
On a psp with firmware 2.80 or lower:
MP4-SP videos must be placed in <PSP's drive letter>:\MP_ROOT\100MNV01 and named M4V#####.MP4 (the # being any numbers you wish, I believe there must be 5 numbers though)
AVC videos must be placed in <PSP's drive letter>:\MP_ROOT\100ANV01 and named MAQ#####.MP4 (the # being any numbers you wish, I believe there must be 5 numbers though)
If your psp is running firmware 2.81 or above, all videos can be placed in <PSP's drive letter>:\VIDEO and named as you wish.
If your videos still wont work you could try running them through
PSPVideo9 using one of the profiles I attached in thread 7 of
THIS thread.