I state the obvious because sometime it need stating - sorry if I offended.
I used to SSH into my systems and run it that way but now have a daily taskj scheduled that does it (and other tasks) and it's applied many, many updates.
Your ARD Task should report something back (No Updates, or a list of updates applied) - if you are getting nothing back at all *something* is squiffy - can you review the Console logs on the machines to see what happening?
Sample Task report:
SYSTEM3 (192.168.1.8)
No updates are available.
Software Update Tool
Copyright 2002-2012 Apple Inc.
Regards,
Shawn