Quicktime player won't open videos on desktop. Any suggestions?

I've had some issues with video I have shot on my iPhone 5c and after syncing with my iMac (Sierra 10.12.3, the videos will not open or play. A little back "converting screen" starts to show up, but quickly disappears. I contacted All support and so far no help. We did a screen share thing, and also tried the new user trick to find the problem. Still no help. I can't delete and reinstall the QT player either since it is part of the system. I've also noted that I have QT 7 Player that works on SOME of the videos but not all of them. Looking for suggestions as to how to solve this. Thanks.