Hardware (Monitoring) resources are not being discovered

We have Solarwinds with the below environment versions.

Orion - 2014.2.0

SAM - 6.1.0

We have been monitoring the devices like Windows Servers, ESX Hosts, VMs, Network devices, etc.

Some of the servers have shown the Hardware Health Sensor under "List Resources" and being monitored. But some of the devices, though they have Dell Open Manager, etc. but Hardware Health Sensor is not being shown under "List Resources".

What could be the reason, and how come we make them available under Hardware monitoring? Please advise.

Thanks,

Naren.