Have you verified that the clients are seeing managed prefernces? If you go to the apple menu > About this Mac > More info ( or just open Utilities/System Profiler ) Near the bottom of the list under Software there is a "Managed Client" section. This should list all managed preferences that the client sees. If the setting is listed here you know the client is getting the preference. If you see nothing here then you need to find why.
Not all settings can be managed by WGM in the "always" domain, maybe move it to "often" and see if it works there. The "often" domain is the one that is supposed to be the most compatible.
If you are comfortable with the command line you can use "dscl" commands to see the actual MCX settings - sometimes the WGM GUI does not tell the "whole truth".