Controlling slides shared by another participant

Hi,

In a meeting this morning I shared my screen as host and then assigned slide control to 3 co-hosts.  Later during the meeting we were joined by another co-host but I was not able to assign slide control to him.  His name did not even appear as an option. 

Any tips on how to make this work or why it didn't work ?  Bit of a novice with this functionality :-).

thanks