This also happened with Top 7 must sees. I am very disappointed about that because I longed to hear once more... "DOES THIS CHAIRLIFT MAKE ME LOOK FAT?"
Have you tried making the file name a little shorter?
That sometimes works.
That's about all I could find on Google apart from (oh dear) codec problems :
MP4s can be a little fussy at times (I just convert them anyway)
see
http://www.pspvideo9.com/forums/viewtopic.php?p=28507&sid=55bcc170ae3cdf16555b93d65d444d38