I'm no expert, but my first idea was an app is stopping this from happening.. If you can put your phone into Safe Mode, instructions how to do this are on the Web. This will isolate everything but the phone, and see if the screen dims. If it does, it's and app. If it doesn't, try clearing the System Cache. If that doesn't clear it, then it's a factory reset. But I'm sure it won't come to this.