4th Build agent not connecting, install seems correct (Linux)

I've installed a Linux build agent before, successfully following the instructions in the documentation.  However, this time I'm not seeing a connection or even an attempted connection.  I had suspected it was because I had maxed out my build agents, but I have bought a license for a 4th agent, and it is not showing up.  I have tried installing it on a different Linux machine to the same result.  My build agents will not connect to my build server.   I have tried both the manual process which I had been successful with before and the "Push Agent" process.  I get errors during that process, specifically missing log files.  I can confirm the build agent(s) are running, they just won't connect.  The IP address for the server is correct.  The build agents are running just fine, attempting to start them using `agent.sh start` fails because there's already an instance running.  I have restarted the build server to no effect.  This is a work stoppage.