Build Agent slow to register after reboot

We are running an older version of TeamCity at the moment, but hope to upgrade soon and we are fully licensed with a full enterprise licence. We're running version 9.1.6.

The problem is that of 4 build agent servers, one of them after a reboot will take a long time, between 30 - 60 minutes or more before it's registered again with the main TeamCity Server. Is this a known issue, which logs should I be looking at to identify what's happening or not happening and how might I resolve this? Thanks.