Hi,
Start by looking for any errors or warnings in the Operations Manager event log, this is the first place look at when troubleshooting SCOM.
Please flush the management server Health service state and Cache
- Open the operations console
- Monitoring View
- Navigate to "operations manager" --> "Management server" --> "Management Servers State"
- In the Management Server State windows, select the grey state management server
- On the tasks panel, select "flush Health service state and Cache"
We can also refer to the steps in the following link:
https://learn.microsoft.com/en-us/troubleshoot/system-center/scom/troubleshoot-gray-agent-states