Quicktime only plays audio when streaming movies from iphone

Quicktime works playing videos I recorded with the iPhone camera but anytime I try to stream a Netflix or Amazon prime movie from iPhone to my MAC (because for some reason you can't download those movies to a Mac....still) it only plays the audio of the movie, no video at all. My phone screen also goes black, but still plays the audio. Both my Mac and iPhone are completely up to date.

There are many questions about related things on quicktime in the help forum. One community specialist, Jameson.h. suggested starting the Mac in safe mode. I did that, and quick player wouldn't function at all when I told it to use my connected iPhone. It said "could not complete operation". I had to quit the app to try again. No luck. Any ideas?