Video taken on iphone won't open on mac

My Mac is El Capitan 10.11.3 and i'm using an iphone 8 to record video. However only since yesterday has it stopped letting my play the videos on my laptop after importing. Only 3 weeks ago it was working perfectly fine. I did see another post the other day with a supposed solution (one for changing your settings before taking the video, but seeing as i had already taken them it wasn't useful for me) saying to change the photo settings so that when it imports it converts correctly. however this didn't work for me as when i transferred them again it made no difference. WHAT CAN I DO!!

In addition, at first i thought the issue was to do with the fact i was using airdrop to send them over. so i tried drive instead. When i upload these new audio and video files to google drive (and only these files, every other file on there is okay eg. images taken in photobooth and then uploaded are unaffected) i can't download them back and i receive an error message unless i zip them up and download them.