Do you have NAT between the system and your SIM Server? SNMP has problem to handle network address translations.
If it is possble I would test snmp access with allow any system in the security property of the SNMP Service.
I also would use the ip address of the sim server and the managed system in the access list. After you get a SNMP ID back you can run the identify system again.
If your managed system is not on the same subnet as your sim server you have perhaps a router which does not forward the SNMP traffic. Check with your network people or use wireshark and compare the answer of a good system and your not reachble sytem using the SNMP system type check.