Active backdrops not working with all my mp4s

[Lancer]

Hi,

I have two different 2K mp4 videos made with my iPad. One doesn't work and I can't figure out why. Both files are about the same size at 25GB and a run time of 20min.  The one that doesn't work, I've shortened to a 5 min, 500MB video and it still doesn't work. I then shortened it to a 1 sec video and it *worked*.  A two min version doesn't work. 

What's happening here? I'd like the video loops to run 10-20 minutes.  I'd settle for 2-5 minutes. But this one video only works as a 1s video.  Has anyone seen this before?