"Display Log-in Window as:" is greyed out

I am running 10.7.2 on a new MacBook. I've created Admin accounts/users and in the first I chose "Display Log-in Window as: Name & Password". Now all subsequent accounts/users are set that way and no matter what I do [including unlocking the lock at the bottom of the window] I cannot switch any of them to "List of Users". PS. This machine was not set up from an image.

Thanks for any help with this.

 

Thank you for your reply. Actually two of the users are Admin, but even when I log in to the original Admin user I do not have access to the radio buttons to change the setting??? It's on all of my new MacBooks as well.