Using Active Directory

I am working with version 8.1 to use active directory.
The web server is configured for Windows authentication.
When I create the application, I can retrieve the active directory listing to associate the application administrator to an AD account.
However, when I attempt to log in, user authentication fails. For the user name I tried to use domain\user name and just the AD user name. Same results.
What could I have missed during setup and/or how to troubleshoot to determine if error in my code or active directory?
Many thanks

Ray