'Span monitors' greyed out, not working

I have a 3 monitor setup and did 'span monitors' on it, which worked. After a reboot (and I think nothing more), only the middle (primary) monitor has a panel, the other 2 don't. 'Span monitors' is selected, but greyed out. I've tried turning other settings on & off to try to enable the span monitors setting, but to no avail.

Version is 4.16.2-1

Cheers for any ideas.