Develops don't show thumbnails in WinExplorere and PS

Recently (with 3.7) I have noticed that some of the develops (tiffs from Canon 10D raws in 16 bit full size) do not show a thumbnail in the PS File Browser. Most tiffs do but the occasional one (less than one in ten) just show the tiff icon. I am working through a large batch if images so the settings are similar for all images (I modify some of the sliders of course).

I can still open the images in PS without problem. And if I re-save the file in PS it gets the thumbnail back.

Has anyone experienced this issue? Or have a suggestion for something to do about it?

I had a similar problem with 3.6 when I developed some images to jpgs. However, in this case it was jpgs and it was in Windows Explorer that some images did not get a thumbnail. (I don't know how systematic this was since I rarely developed to jpgs.)

Odd...