Adding co-organizer issue

Hi
Am hoping someone can help me as I'm going round and round in circles.

I have 2 accounts in my outlook that are my bosses companies.  The first one works fine - I set up Teams meetings - make him co-organizer - no issue.

The second one I make meetings, it adds him but says it can't see his calendar (not sure if that is relevant) and when I try to make him co-organizer there is no one in the drop down list - despite a list of attendees.  If I go to the "who can present" box and say Specific People - he's in that drop down list.  

So why isn't he (or other attendees) in the co-org list?

Thanks!