I have run in to a similar problem.
I didn't read it anywhere in the documentation, although I'm sure it's there somewhere...but...the trick to getting the OCS classes to work with WMI is to NOT connect to a remote machine.
All WMI calls for OCS must be made on a machine that has the admin pack installed. Also, you MUST connect to the local machine (with the admin pack installed) for the queries to return proper information. It MAY work in different scenario's, but in the one I just described, it's gauranteed to work all the time.
Note that because of this caveat, you have to query from within the same domain.
Hope this helps...I scratched my head with this for a while before getting the proper tip.
Cheers