Hi,
check if the Orchestrator DB ist working (https://social.technet.microsoft.com/Forums/en-US/096fdd1c-bef0-4204-8e84-487e2d1a4808/runbook-not-running-and-wont-stop?forum=scogeneral): Perhaps the DB or tempDB is out of space?
Can you also check in tab "Log History" if an instance is stopped when you click ob "Stop"? Perhaps the Runbook is in the queue for starting caused by an external trigger or "Invoke Runbook".
Regards,
Stefan