No issue with nested groups in Administrators. We just need the agent action account to have admin rights to the server.
The _Collector Service Status view is CRITICAL - if you see no Collector there, then there will be no Hosts, no VMs, no Datastores - no vSphere objects. Discovery of Collector Service is the first stage of all other discovery. The Collector service must appear in the view and it must be "monitored" status.
You can try restarting the SCOM Health Service (Microsoft Monitoring Agent) on that server - that can force discovery of the service.