Hello Paul,
If you close out of the Device View and then go back it does it present data?
If it still does not work then please try querying that device with the following FlexView:
XOS System Power
You can use the FlexView by right clicking on the device and selecting View > FlexView
To get notifications of power supply failed/normal you will need to setup traps on that device. To do that you should be able to right click on that device and select Device > Register Trap Receiver. You can verify that the correct SNMP credential is used for this under Administration > Option > Trap.