In LrC, not all previews are populating

LrC ver 11.41 and 11.5

macOS Monterey 12.5.1

In library mode, when I select a folder of photos, previews (thumbnails) appear randomly with other previews grayed out. If I click on any grayed out preview, the photo appears. I have deleted the preview folder forcing a rebuild but this does not cure the problem. 

Any advice appreciated.

Des