The message states the desktop (the agent) is unreachable, it means Connection Server(s) can't connect them.
If you are on separate vlans, check routing and firewalling.
Check desktop can resolve servers name and vice versa.
Check desktops actually have an IP address (how many times problem was just the DHCP not working properly or lease was too long so DHCP ran out of addresses ?).
Do you use a KMS ? Are desktops activated (microsoft wise) ? If you are using the grace period, you have to modify your master's registry to not check for activation to be done.