Health State for Root Manager shows as "Not Monitored"

 for no reason, health state for Root Manager shows as "Not Monitored", as consequences, it stops monitoring the servers with agents installed and pushing out agents to new servers.

it's 32-bit Windows Server 2003 R2 Ent with SP2, running SCOM 2007 SP1.  to trouble-shoot, I've done:

• all Operation Manager services that are running on RMS; they are in running states - tried restart all the services
• imported all MPs
• tried "Repair" agent and "Reset Health"
• tried all suggested fix listed in posting here.

any other suggestions?  thanks in advance!

 Are you monitoring network devices?

I have read that their SNMP settings can cause this sort of issue. Although I have not seen it myself as we don't use SCOM to monitor network devices.