There are a number of things that can cause this.
First, check the HP SMHP on each server and check that it can see the server hardware details.
If not, you are most likely missing the WBEM software.
In addition, you may need to swap the provider on the homepage from SNMP to WBEM or visa-versa.
You also need SNMP running on the server.
On top of all that, don't forget firewall ports need opening between your SIM server and the SMHP servers.
If I remember rightly, some servers also require a Power Manager driver to be installed. Can't remember the exact name of this off the top of my head, but it has something to do with the interface to iLO.