They do have JPG previews in them. Note my "Update" that when I shut down PM and start it back up,they are just fine. Also note that this occurs randomly on only a few images.
What I think I have noticed, but it is hard to confirm, is that at times I can view an image with no problem, but when I go back to it, the problem occurs, in the same PM session.
The thumbnails look fine, are these not also generated from the JPG preview?
From the behavior I am seeing, sending you a NEF won't do much good I'm afraid, as the problem occurs on different files when I reopen PM.
I just did the following test:
1. The problem manifested on 2 images out of 11
2. Closed PM and restarted
3. Checked thumbnails, all showed the "Zoom" button as expected
4. Opened the 1st image as Preview
5. Used the + key to Zoom, then the - zoom to come back to 50%, worked as expected
6. Used the Right-Arrow key to move through the next 10 image
7. 2 different images now exhibit the behavior, and their thumbnails do not show the "Zoom" button.
Needless to say, this makes it very difficult to preview images with any consistency. I will verify, this evening, that the same occurs on my laptop.