Multi Variants - drop down products not working - Video

[Gingerweb]

Hi.
I have setup multivariants many times, even on this particular site and not had any issues but all of a sudden it isnt working for new products using the same multi variant custom field.

Here is a 3 minute video of the setup process : https://share.getcloudapp.com/kpuKObpm

I dont think i am doing anything different to how i have done this before - is this a bug or can someone point out a glaring mistake i have made?

thanks