We upgraded to 18.104.22.1684 from 22.214.171.1249. All agents except 2 got the update and show in Agent Overview after the update / web server restart.
I am thinking that these agents may not be connecting to the collector now.
Also, what is the best way to check that an agent it connected it collector?
If you have access I would log into the servers with the agents that aren't showing up and see what is recorded in the agent logs. This should tell you if it's a communication with the collector issue or something with the agent itself.