I think i might have found a solution to it. Try experimenting by going in a dim room or a dark room(better), set auto brightness ON and adjust the brightness bar to the MINIMUM. Next, switch on the lights in the room and you will see the brightness bar gradually increases by itself, means it's going to the right side.
This is to show whether the auto brightness is working or not. If it is moving, yes, it's working. If not, you know what to do.