An agent that is not seen in Databases Dashboard is an agent that is not working, for some reason.
Agent Status shows that the Agent is activated and marked in green, but behind the scenes the agent is not functioning.
A database agent must have monitored successfully at least once to be visible in the databases dashboard. If an agent no longer appears in the dashboard by the agent is shown as acting in the Agent Status panel then the most common reason for an agent not to function as expected is topology duplication.
In this scenario, the agent may have been created more than once.
For example, the user has created the agent by using Database Installer, then deleted the agent from Agent Status and created it again by using the Agent Status Properties (ASP).
If not properly deleted from the topology, the agent will maintain two topologies:
Note that The Topologies are created in the FMS process and agent is running on the FglAM process. That explains why the agent is marked in green in Agent Status, while there is a failure on the FMS side.
Another reason for the agent to be shown as active in Agent Status and not shown in Databases Dashboard can be related to NTLM2, Microsoft security protocol.
Best practices would be to create the agents from Auto Discovery only.
Auto Discovery will run validation checkup before allowing agent creation which may prevent agents duplications.
If an agent is green in Agent Status and is not shown in Databases Dashboard , suspect topology duplication.
If there is more than a single Agent under Instances, it means that the agent is working but the data is pushed to the wrong tree.
And if there is no data, the agent will not exist in the Databases Dashboard.
Next step would be to delete the topology for the agent in the proper way: