Mobile friendly 'viewport meta tag' working one main-site only.

I have placed the default code into my 'Header' and thus have passed Googles 'Mobile Friendly' test! on all my pages!!!

But, although the 'viewport meta tag' is working one main-site ; it is not working on any of the multi-sites!

This is also the case on several different sites that I have!

Perhaps this is a similar phenomena to 'plugins' that need that 'little extra' before they will work on multi-site?

Any suggestions, please?