Use this tool to monitor GUP health. It will tell you all you need to know about your GUPs:
https://www-secure.symantec.com/connect/downloads/new-sep-content-distribution-monitor-gup-health-checking
Check these links to verify which clients are/are not getting updates from your GUPs:
http://www.symantec.com/business/support/index?page=content&id=TECH169006
http://www.symantec.com/business/support/index?page=content&id=TECH188574