Slide over to the widget where you can select between brightness levels and verify that it's on "A". Also make sure that the brightness is actually changing. The best way I could make sure mine was working was to go into the bathroom and turn off the lights, then turn them on. I will point out that the actual difference between dark and light conditions is minimal from what I can see. It seems like more of a low setting with 15% difference applied by sensor feedback.
I am going to guess that you have likely checked to make sure it's on auto and it's actually changing brightness by manual control. If you did check both of these things, then I'd say you might have a faulty ambient light sensor. I'd suggest taking it in for replacement if that's the case. If you can't tell a difference between light and dark rooms, then I am going to guess that they won't be able to show you a difference in the store and would have no problems with replacing it.
