Hello Ann
Thanks for you interest. It appears that the difference between the two files (our of hundreds) was a coincidence. Most of the files that are not displaying are UHD. (I only shot a few HD files accidentally when learning to adjust settings on the tiny Osmo Pocket screen.)
If it might help, I could provide MediaInfo on dozens of UHD files from this shoot in including consecutive shots where one is displaying and the other is not. (I've attached one more UHD VideoInfo file of a video that displays perfectly. (Several hundred do!)
I'm running Adobe Master Collection CS6.
This still seems like a real mystery to me.
Thanks again for your help.
All the best
Marcus
Thanks again for your help.
Marcus