MSSQL Monitors - The Monitor did not respond...

MSSQL Monitors - The Monitor did not respond...

I've enabled the MSSQL Monitors for my SQL server but I don't see the graphs for the monitors that I've enabled. When I tried to test the monitor, the following message appeared:
The Monitor did not respond to the test request

My resource monitors for the same server is working fine.

Any ideas on how to resolve this? Thanks.