I've found AppGameKit is *VERY* finicky with regards to supported video-formats on Win7. With the exception of .WMV which it'll gobble up without complaint. On Win10 mp4 files encoded with h.265 will also behave very nicely, though this format is not supported on Win7 straight out of the box. With h.264 it is a crap-shoot on both Win7 and Win10. Some play nice, others, not so much. This is regardless of how the file plays in a video-player.
Speaking of, all mp4 files encoded with h.264 encoding I've thrown at Android and iOS devices play perfectly - though on iOS there are trouble with keeping track of play-duration and setting a specific playback point that isn't the beginning. Using h.265 encoding (better quality, smaller size) will only work on Android 5.0 and later, and iOS11. For OSX, I have no idea, I just use that abomination to compile and distribute onto iOS...
Linux - well, video isn't supported. Though Paul have put it onto the todo-list. Unfortunately this means that I've spent quite a lot of time using Win10 lately developing the app I'm working on, as it is mainly centered around video.
But if you're stuck on Win7 - try repackaging your video files to .WMV.
For my project, my server-side video-repo is h.265 encoded - with some code on the AppGameKit client to detect Win7 and iOS and feed those h.264 videos encoded on the fly on the server from a h.265 file before transfer. Not a biggie for my application, as the videos are quite small (average ~150-200kb a piece). This works fine for now, but before release I'll have to keep two video-repos to save on CPU usage on the server. Storage is much cheaper than CPU use on just about any cloud host
