Hello, Over time I've noticed that some thumbnails of movies disappear. Will useing the lock feature fix this problem?

A database repair probably won't restore your missing thumbnails, but the following procedure will:

On the right topside of the thumbnail section, you see an icon with an arrow on the right. Click on the arrow, and select view as thumbnails. Now click on the arrow again, and select rebuild thumbnails. That should do the trick.

Quoting cvermeylen:

Quote:
A database repair probably won't restore your missing thumbnails...

True.  Still, it's a good idea.  Thumbnails don't normally simply disappear.