Poster Frame in Photos not working

I just played with some videos, and it's working for me. You didn't say if you're using iCloud Photos or if you have "Optimize Storage" turned on-- that information would be helpful.

To narrow things down, try these steps:

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. You would need to move your Library out of your own user's folder up to the general "Users" folder. 

• If you're not using iCloud Photos, try to rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…