Credit to HawkRidge.
My Windows Explorer thumbnails were showing in some instances and not in other instances. Struggling with this for a long time. Had fixed it a couple years ago but it came back (maybe a different issue, but same problem).
Looked here on forum, nothing. Google, nothing.
VAR helped out, we figured out it was fine on a local drive and not so much on the network drive. The tech found this, option two solved it!!!