I saw in Appmon 2 agents with the " connected(issue:instrumentation is not available)"
When I analyzed the mainframe CTG log I verified that the agent made the connection with collector but in Appmon the CTG agent is with "Connected(instrumentation is not available" status
If you right click on that agents and click details, you should see detailed info why the instrumentation is not available.
Usually, that the agent didn't match any system profile or licence exhausted...But maybe is other one!
Check this and you will get the desired information to take further actions!
Hope this helps
You can also check Agent logs, there can be info if for example those nodes has problems with connecting to collector. In such case if collector can't finish instrumentation, agent can be connected but not instrumentented. Check @Arturo Mondelo R. sugestion, if there will be to less information, go to System Information and check logs.