I have the solution, without necessary restart, just a quick method and it works.
First: go in finder to the directory that doesn't show the thumbnails. Then go to Library ("Go" menu in the finder, hold down the "alt/option" key on your keyboard and the option to click on "Library" will appear (by default the User Library is hidden from view)).
Then select the two files:
com.apple.finder.plist
com.apple.finder.plist.lockfile
and right click them straight to the trash.
After that:
force close the finder 2 times immediately one after the other and that's it!
Good luck 🙂