Sounds like an encoding issue.
The container may be mp4 but what is the encoding specs of the files?
what program did you use to convert the files?
the other files you added. do they play just fine? were they converted using the same program?
just how large are the files in question? 1gb? more?
what player on your computer are you using? and is it PC or MAC?
what app are you using to play them? just the stock app? try: