If a mac address has machine authenticated, by default the result is cached for 24 hours. The parameter that controls that behavior is under Administration> Server Manager> Server Configuration> Click on Server> Service Parameters> Select Policy Server Service:
Alternatively, you can clear the machine authentication cache manually by clicking on Administration> Server Manager> Server Configuration> Clear Machine Authentication Cache:
The reason the cache is in place is because machine authentication only happens when the machine is sitting at the ctrl-alt-delete screen, either because it just rebooted, or because someone logged out. Many people do not log out of their computers, however and just lock them. When they wake their computers up, machine authentication does not take place, so CPPM will think that it is not a domain machine. The machine authentication cache stores the previous authentication and renews it, every time there is a successful authentication, so that the user does not have to reboot or logout of their machine to demonstrate that this is a domain machine. I hope this helped.