I am having a problem getting my account lockout policy to work.

I have a setting in my Default Domain Policy in my Server 2008 Active Directory domain configured as follows:

Account Policies/Account Lockout Policy
Policy                                                        Setting                                    Winning GPO
Account lockout duration                       30 minutes                            Default Domain Policy
Account lockout threshold                     5 invalid logon attempts      Default Domain Policy
Reset account lockout counter after     30 minutes                            Default Domain Policy

I have confirmed the settings are active by running the Group Policy Results Wizard on the domain controller for the machines and logon IDs I am testing.  I have also confirmed the settings on all machines using the "net accounts" command with the following results:

C:\>net accounts
Force user logoff how long after time expires?:       Never
Minimum password age (days):                                 30
Maximum password age (days):                                90
Minimum password length:                                       8
Length of password history maintained:                  4
Lockout threshold:                                                      5
Lockout duration (minutes):                                      30
Lockout observation window (minutes):                  30
Computer role:                                                            WORKSTATION
The command completed successfully.

However, when testing, the accounts never get locked out.  I do six invalid attempts and the OS pauses for about 15-30 seconds, then it lets me try again.  On one account, I tried 31 invalid attempts within about 3-5 minutes and it still let me login after that.

I have confirmed that this is not confined to a single machine, server, or user account.  The same lockout information is displayed using "net accounts" on 5 different workstations and 3 servers.  All 8 machines tested do not lock out the accounts used.

Any ideas on the potential cause or hints on where I can look to troubleshoot this?

Thanks!