Unfortunately Cmdr Heatherly called “no joy”, and went below the hard deck. Thanks for the suggestion about Remuxing it though.
It would be nice to figure out why it doesn’t generate thumbnails, even if its not something easily fixed. Its only one file that does it, so I guess I’ll have to learn to live without them.
I think I remember reading something in another thread when searching about Key frames being missing, but I think that worked fine for chapter thumbnails, it was just the video thumbnails that were missing.