I have recently had this problem of thumbnails showing but not expanding, and the triangle appearing. I have tried some of the suggestions to fix this, but it is still a problem. Photos that I have uploaded from my camera seem to be ok, but if I try to view those taken with my iPad or iPhone I can’t expand them.
What is the system version of your iPhone and iPad? And the system version of your Mac? Are the iPhone and iPad images in the space saving HEIF format? Then your Mac would need a system version of macOS 10.13 or later to be able to handle the images taken with the iPhone or iPad.
See Using HEIF or HEVC media on Apple devices - Apple Support
Warning triangle in photos.
