I find the thumbnail is slow to update and often doesn’t update for days (at all?). A work around has been to choose “Restore default thumbnail” and then “Set as thumbnail” on the desired frame. The thumbnail has become an important feature to me and I use it to convey information to my team. The inconsistent thumbnail updates are a little problematic.
Suggested solution: an option in the right-click menu for the file to “Refresh thumbnail”.
Or just a more consistently working thumbnails system…
Well, there is that option too. 
Until this is live, you can force refresh your thumbnail by removing and re-setting the frame as the thumbnail. 
That forces immediate changes.
That usually works but I have had times it didn’t.
Same problem, and none of these solutions are working. I’d love a more reliable way to update the thumbnail
Slightly related: I have a stubborn file that won’t show any thumbnail at all, even if I set a frame as thumbnail and then reset it again. I’ve tried duplicating and making edits to the file, but it’s still just gray! 
Possible solution:
Since the file’s frame that contains the cover/thumbnail is presented as a “special” frame once you set it as a thumbnail, maybe ONLY this frame should have a refresh button beside it’s custom icon:
