If your clients are running on XP, you may have Fast Logon (http://support.microsoft.com/kb/q305293/) turned on.
Use the following policy setting to turn off Fast Logon Optimization:
Computer Configuration\Administrative Templates\System\Logon\ Always wait for the network at computer startup and logon
With this enabled, an XP client behaves in the same manner as a Wi2K client at both system startup and at user logon. The drawback is, the logon process may take longer.