LDAP Authentication

I'm trying to get the LDAP module functioning properly. It is configured and tests fine, but when a user enter their credentials, nothing happens! No error message, no 'bad password', just a screen reload and back to the home page. Any thoughts?

Will Drupal pull the users credentials from Windows (this is strictly intranet based)? I'd prefer that users are signed in automatically and not required to enter their credentials.

Thanks