Files inconsistently not appearing in photo browser/thumbnails

On my MacBook Pro, whether I am in PL5.1 PhotoLibrary or Customize, some files (Panasonic S1R RW2 files) are not appearing in the Photo Browser/Thumbnails. Usually there are several gaps of 2 or 3 photos. The files show up on Finder. It is inconsistent in that the files will appear sometimes but not other times, and it’s not always the same files that are not being displayed. .A little while ago I had a photo that wasn’t displaying in the Browser on one occasion appear after closing/reopening PL. After subsequently closing/reopening again, that photo failed to appear in the browser again, even though that photo was actually displayed on the edit screen after reopening.

Have you checked the filtering options?

Yes, repeatedly.and reset them.