you can read all about problems with mp4 files here…
support team are aware of problems … hopefully a fix in the next firmware
p.s. 90% of my mp4’s play without issue … one’s that don’t, i remux them to mkv using MKVmerge, then they are ok
MKVToolnix (includes MKVmerge) http://www.videohelp.com/tools/MKVtoolnix
hope it works for you