SCOM Agents are in not monitored state, But Health service watchers are healthy

Hi Experts,

We are struggling with very odd issue in our SCOM 2012 environment.  We are monitoring around 45 DMZ servers using GW, In which 37 agents are in Not monitored state.  When I checked in Operations Manager event log I didn't find any error events and everything seems to be perfect, configuration are updating properly in Health state folder.

I found that Health service watcher for respective agents are also healthy.  I tried Health service flush, renaming Health state folder, but they didn't helped to fix this issue.  I restarted all services in Management servers and restarted MS& GW also but still Agents are in not monitored state only.

When I checked for availability report, It showing that Monitor disabled.  Kindly provide your valuable suggestions to fix this issue. 

Thanks in advance.

Br,

Karthick Malayalan