You notice that you have initiated an endpoint discover scan and the scan stays in "Starting" state for hours and does not start the scan.
Endpoint discover scans run serially on the endpoint agents. Therefore if a scan is already running on an agent and that agent is also a part of another scan target, the subsequent scans will stay in "Starting" state until the first scan has completed on the agent.
You need to review the scan target configuration to ensure that the agents are unique to that particular scan target to avoid this kind of a situation. Also, if only the Endpoint servers are specified in the scan target without any agents, the scan will be triggered on all the agents reporting to those particular endpoint servers. Thus if you are trying to start any other scan target which has the same set of agents this scan will get stuck in "Starting" state until the first scan completes.
This behavior is by design. Ensure that no existing scan is running on the agent before initiating another scan.