If your MP4 videos are not playing when you first load them on a page it is most likely due to to the file encoding. The file will play, however, the whole movie will need to be downloaded first. The MOOV atom of the file is at the end, so it cannot be progressively downloaded.

There are lots of different tools that will fix this:
http://www.dvbportal.de/projects/mp4muxer/
 
You can see about a coder potentially qt-faststart on all of your mp4 movies on your server. Ask us for a reference list.

Alternatively, anything made with our video transcoder plugin will have the MOOV atom at the front. More information here:
http://www.elevatedx.com/transcoder.php

You might also want to look at adding a streaming service if you do not plan on using our transocder such as Wowza|Red5|mod_h264