Jenkins agent not connected to master
If the Jenkins agent is not connect to the master you can have a look at the Jenkins Nodes UI and see if it’s possible to diagnose and solve the problem from there.
If there doesn’t seem to be way to solve the problem it may be necessary to restart either the agent or master process:
$ ssh ci-agent-<n>.ci $ sudo service jenkins-agent restart
$ ssh ci-master-1.ci $ sudo service jenkins restart
Caution that restarting the master process will cause all CI jobs to be temporarily suspended and all agents will need to reconnect.