I logged into one of my OpsMgr environments and on the Administration space I ran across an interesting issue. A functional management server which considered itself to be Not monitored (see below).
Reviewing the event logs on the management server there was nothing interesting so it wasn’t obvious what the issue was… Until I looked at the management server from the monitoring pane within the computers view. Ah-ha! Maintenance Mode!
Once this was removed from maintenance mode all was good again from a Administration \ Device Management \ Management Servers perspective.
Summary: A previously Management Server shows as Not monitored – check to make sure that someone didn’t put it into maintenance mode.