After importing video files into Organizer 15, I noticed that many thumbnails were missing for .MOV files and .MP4 files.
I just discovered that installing the last version of Quicktime 7.7.9 solved the problem. There was some earlier version installed on my machine before that.
I used the "Update Thumbnails" command to rebuild the thumbnails after selecting the videos that were missing thumbnails.
Here's the Quicktime download for the US local: https://support.apple.com/kb/DL837?viewlocale=en_US&locale=en_US
Apple no longer supports Quicktime, but I need it.