When I try to shut down my iMac when more than one user is logged in; I get a dialogue box warning me that I may lose any unsaved. It asks for an administrator name and password. So I entered my name and password; same one I use to log into my iMac when I start it up. I then get another dialogue box saying I must enter a valid username. Well my settings is an administrator so I can't understand why it won't accept my login details Any ideas how to resolve this?