Printer deployment via Group Policy not working on a single system

Is the driver already installed on the Clients? I am thinking that perhaps the users do not have permissions to install the initial driver and that is causing the addition to silently fail.

update:

If they don't exist already on the client you need to ensure that the point and print restrictions are set to allow silent driver installation. In a mixed environment (both win 7 and prior clients) you need to set it in both User Configuration\Policies\Administrative Templates\Control Panel\Printers and Computer Configuration\Policies\Administrative Templates\Printers.

set it to not show the elevation prompt

See if my previous answer on this helps: Mapping printers using Group Policy Preferences; works on Windows XP, not on Windows 7 x64