So recently I had an issue where thumbnails of videos weren't loading at all in the Zorin file browser (officially known as Nautilus.)
It's probably an issue with my video player, since I use mpv instead of VLC. However, I found this trick in an Ubuntu Help thread:
sudo apt install ffmpegthumbnailer
This package seems to handle thumbnail generation of pretty much any kind of video!
Going into the Nautilus preferences (located under the menu with the three-line button) and raising the "Thumbnails only for files smaller than" value (under "Search & Preview") to something like 50 MB would also be a good idea.
After you do all that, hit refresh in your folder and watch all your videos gain thumbnails again! Wow!