These servers have sent a heartbeat but have failed to respond to an appropriate query from a different port. This can happen when:
Server is firewalled and all inbound packets are dropped. Try reconfiguring your firewall to allow packets to that process or port.
An intermediate router implements an Address/Port-Dependant-Filtering NAT. Try setting up port forwarding.
Outgoing connections are asynchronous with regard to port forwarding. Such servers will show with arbitrary ephemerial ports. Docker: you can supposedly work around this with --net=host.
Plain and simple packet loss, servers in this state for less than 5 mins may still be fine.
Server crashed or was reconfigured before it could respond.