you need to look at how the clients (or groups) are set to receive updates.
If you have 2 groups and one of them is not receiving updates then it could be that the groups is not configure correctly.
Take a look at this article and see if this resolves your issue.
http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/2007091122402048?Open&seg=ent
Also this flow chart is a great way to help you isolate your issue.
http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/2009082702000348
Test and post back