Does the 'big buck bunny' mp4 file that comes with Adapt work in this situation? If so, and the mp4 file you have created does not, then that certainly suggests it's something to do with video encoding.
If the big buck bunny video doesn't play either, that suggests it might be a server problem - the first thing I usually check for is the mp4 mime-type being correctly set on the server.
Video is one of those things that seems like it should be simple but actually frequently ends up being a massive pain in the wotsit.
Although HTML5 video has greatly simplified how you actually add video into a page, it has generated plenty of other issues to go along with it - starting from the most basic problem of some browsers needing different video formats (Firefox on most non-Windows platforms doesn't support MP4 for example) to all kinds of small things, such as:
- the 'moov atom' being in the wrong place in the video file (https://stackoverflow.com/questions/5058917/mediaelement-js-flash-video-wont-play-until-fully-loaded)
- security/authentication applied at the server level stopping videos loading in iOS (but only for our client, not for us!!)
- a client's network blocking playback of videos via IE's standard HTML video player (yet same file would load fine using Flash-based video player)
- Firefox suddenly not loading .ogv files for videos served through Moodle, and then several days later suddenly working again... (possibly caused by a bug in a release of Firefox although the release notes claim there were no changes to video handling)
- Videos not playing in Android when served via Moodle because you have the Moodle setting 'only http cookies' enabled (https://tracker.moodle.org/browse/MDL-44849)
And plenty more which I can't recall offhand!