MP4 files not working in Quick Look OSX Mojave 10.14.6

Something is preventing MP4 files from playing with Quick Look.

MOVs play fine when I hit the space bar, jpgs, pngs, pdfs etc all show up correctly but MP4 files: just a black rectangle with the filename at the top of the window.

My mbp used to be able to do it but has been like this for a few years now even across OS updates from a few versions back.

My partner has an almost identical machine but running 10.12 and it previews MP4s fine when hitting the space bar.

I've tried many fixes, from Safe boot, to Terminal commands and I'm still stumped.

I must have spent a considerable amount of time waiting for VLC or other apps to open just so I can preview MP4 files over the years instead of that quick tap on the space bar. I've also spent a fair bit of time trying to track down a fix.... but I'll give it another go if anyone has any suggestions? TIA :)