Thumbnails not displaying in Collection

I recently added items to a collection that include 2 images each (1 large and 1 thumbnail). The thumbnail images preview perfectly on items 1-17. Items 18-22 do not display image previews on the collection list. I do not see what would cause the images to not display after a certain number of items since the thumbnail images are all the same size. Any suggestoins will be much appreciated! Thank you.

My guess is that there is something different about the original files for item 18-22 that prevent a derivative image from being made. Maybe file size, or format quirks. You might try turning on error logging, to see if the system gives a message about a problem trying to create those derivatives. (I notice that in items 1-17, some files also don’t get derivatives, so that could also be related).