Hello,
To collect OS and other info SIM uses the Bi weekly Data Collection Job. You can run a datacollection by hand and see if it succeeds. This job used the credintials that work for the server. This would be SNMP for a Linux server. You could try to restart the SNMP and SNMP agents, then run an identification. If the identification runs like i should you can then rerun the datacollection to see if the OS version get's updated.
Most of the time the problem is that the SNMP and/or SNMP agent on the server is broken. If the server is a Gen8 or newer the OS info is read from ILO by using the AMS agent. IS the server a gen7 or older or Gen8 or newer?