After the vCenter discovery, if the VMs and Hosts are missing, query the below table and check
select * from vwentitymomapping where entity_type = 'VirtualMachine';
select * from vwentitymomapping where entity_type = 'HostSystem';
When the MOID is -3, Virtual machines have not been selected by the user to be monitored. By clicking on 'Update inventory' option in vCenter device snapshot page, list of virtual machines and hosts which are available will be listed. Select the VM's which have to be monitored.
When the MOID is -4, check whether their DNS resolves, if not update it from vSphere client › host property
When the MOID is -5, that's marked as "deleted by user" so OpManager won't be mapping it.
We have to run this update query manually and restart OpManager service.
update vwentitymomapping set moid = -4 where moid = -5;
Now automatic discovery will initiate during every restart.