Black box around videos when on Youtube..but intermittent

Hi guys,

Apologies probably a basic question but my 5 minute Google search did nothing and I'm thinking of throwing my PC out of the window 🙂

I have uploaded about 60 videos onto Youtube in the past few days. Most of them are fine

https://youtu.be/GS9VC0tx6b8 

It's all shot on the same cameras, with the same settings as far as I know, and I just can't seem to find a straightforward answer as to which of my settings is wrong. I have about 100 more to upload, and am under quite a bit of time pressure, so could really do with figuring this out in advance of doing that.

thanks very much.

Peter