Verify the hostname as seen by the ambari agent and server are the same
1. In the /var/log/ambari-agent/ambari-agent.log , search for socket.getfqdn, sample line below
hostname.py:106 - Read public hostname '<servername>' using socket.getfqdn()
2. Get the list of hosts from the api
http(s)://<ambariserverhost>:port/api/v1/clusters/<clustername>/hosts
Verify the FQDN <servername>