When AppDynamics agents fail to connect to the Controller, it can stem from misconfigured connection details, network firewalls, or SSL certificate problems. The first step is to check the agent logs, which typically indicate the root cause of the failure. Verify that the Controller’s host and port are correct and accessible from the agent’s environment. If SSL is enabled, ensure certificates are properly configured and not expired. Firewall and proxy restrictions should also be reviewed to allow outbound connections. Version mismatches between agents and the Controller can sometimes cause compatibility issues — updating both components often resolves them. For persistent problems, AppDynamics provides diagnostic utilities to trace connectivity paths, helping administrators quickly restore full monitoring functionality.