[ALUG] No pdf thumbnails

I'd not been having any trouble with pdf's for a while but suddenly no recent pdf's show a thumbnail, just an Adobe logo. I've got Master pdf and the simpler Evince (for when I just want to read something.) In the past, if this happened, all I had to do was change the file name and then a thumbnail would appear. That doesn't work here. (I think I've asked a similar question before but on hunting through the archive couldn't see a clear answer.) Bev