new SEP 12 clients don't connect to SEPM Server

We are on SEP 12 version 12.1.5337.5000

We have had two instances in the last three weeks of newly installed SEP 12 clients not connecting to the SEPM server.  It is as though they don’t see the server.   It involved several newly imaged computers when it happened.   In both cases a reboot of the SEPM server caused the agents to find the server and start communicating.  It will then run fine and talk to everything.

This did not happen with previous versions and we were running 12.1.5 for two months before this started. 

Has anyone seen anything similar?

Thank you