As a first step, check that Resultant Set of Policy (RSoP) is showing the setting as being applied. If it doesn't then RSoP should give you the tools to track down the reason.
Assuming RSoP shows that the setting should apply but it still isn't you could enable verbose Group Policy debugging and sift thru the event log. Create a REG_DWOORD value "RunDiagnosticLoggingGlobal" at "HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Diagnostics" and setting it to "1" on one of the affected computers.
Problems with Group Policy application have, in my experience, almost always come down to unintended security filtering (or other scoping-related issue) or another GPO with a "conflicting" setting taking "precedence".