Screensaver won't turn on despite group policy in XP

My office is currently running XP SP3 on all our desktops and Windows Server 2003 on most of our servers. We have a logon policy in place that imposes a screensaver activation period of ten minutes on all computers, with the added requirement of 'on resume, password protect'. This is set to not be alterable by the user.

Yesterday one of my users reported that despite his computer being left unattended for several hours over the weekend, his screensaver had not engaged and the system did not ask for his password. I checked to make sure he was picking up the policy, and he was. I examined the registry, too- HKCU > Software > Policies > Microsoft > Windows > Control Panel > Desktop > ScreenSaverIsSecure was set to 1 and the time on it was set to 600.

When I came into the office this morning, the user's computer had not engaged the screensaver overnight, nor locked down with any form of password protection. I have since locked the computer manually, but I would like to know why the screen saver isn't kicking in. Multiple apps were running when I checked the machine this morning- Outlook 2007, Excel 2007, Word 2007, Acrobat 9, Blackberry Desktop Manager, and I think Bloomberg Professional, plus several items in the background along the lines of Symantec Endpoint Security and GoToMyPC. If anyone can suggest what needs to be reconfigured or shut down in order for his system to lock automatically, I would appreciate it. Thank you.