Can't import some photos from iphone

I plugged in my iphone (iOS 15.6.1) to my Mac Pro with el capitan (10.11.6) to import photos as I usually do. The mac opened iTunes but did not open the app photos? So I opened photos and clicked on import - some of the photos showed up normally in the import window while many others showed as empty grey boxes and I cannot import them. On the iphone the photos show normally. It worked flawlessly several weeks ago. I tried on my macbook pro (el cap) and got the same thing? Any ideas why only some photos show on the Mac in the photos app import window while others show as empty grey boxes (see photo below).