Are you discovering gen6 and below with ilo first?
I usually see UUID for servers when you discover ilo2 first.
Make sure the proper agents are installed on the host and is configured for either snmp/wbem.
If gen8+, make sure the ilo4 is setup with agentless /snmp community string and the server have the AMS installed.
If SIM sees a server with the same uuid during a proper discovery it will merge the two nodes together.
http://h20564.www2.hpe.com/hpsc/doc/public/display?docId=emr_na-c03306157