I'm running out of options, is the firewall of the SIM server active? If so try to disable it just to test that's not the problem.
You could check if the traps directory under the SIM installation has files in it, SIM uses this directory to buffer it's snmp traffic. This directorg thojld have files comming and going all the time. You could also set a Windows server to accept SNMP queries just to see if SNMP is eorking from scans of the SIM server. Just set if not already configered the SNMP to Public and allow SNMP from the name or IP of your SIM server, set ghe datasource to SNMP and look if the System Management Homepage still shows system info.
This is all just for finding out what's wrong you can revert these setti gs when you find out the source of the problem.
Good luck.