How do I generate thumbnails for images in old folders?

A while ago I turned off something in Windows because of weird issues happening... and I think its related to something I noticed today. In old pictures folders I no longer see thumbnails. I just see a white sheet of paper icon (well it looks like that) and the filename underneath.

How do I restore thumbnail generation within a folder and its subfolders? I've already set it to Pictures type...

Hmm, a reboot solved most of the folders...
but not all.
maybe another reboot will finish the job?