Unable to add snmp monitors

Opmanager build 7004 on CentOS 4

I am not able to add the SNMP performance monitors to one of my servers. I know that SNMP is working on the server because I had these monitors added once. I have others servers that I have been able to set-up these monitors without problems.

When I try to added a monitor, like Free Disk Space in GB, it is already selected in the add monitors screen but I do not see it in the Performance Monitors section. I have also tried to unselect this monitor but each time I go back into the Add Monitor screen it is still checked.

Can someone give me some ideas on how to resolve this issue?

Thanks in advance.