I have got 3 Domain Controllers fowarding events and 1 collector collecting Security events from those 3 source machines, they are all on the same Domain.
However after restarting Windows Event Collector, I go to the Collector machine -> Event Viewer -> Subscriptions -> right click the name of the subscripion -> select Runtime Status, I will see all those 3 source machines are inactive.
I don't know how to bring them up immediately, although I still have some some client machines generating events and sending to those 3 DC. But they will eventually turn on, say after half an hour also.