typically there is a blue lettered Fn button that controls keys with blue symbols on them. One of those blue symbols may be brightness up. It is that way on my machine.
other Fn keys are a numeric keypad, sleep button and scroll lock.
pardon me for stating the obvious, but are you pressing the Fn key AND the brightness key?
in any case, try the following:
navigate to the Display Properties. select the Display tab, press the Advanced button.
on my machine there is a tab for the GeForce 8400 GS. Maybe your machine has a different video card, but there may be a similarly named tab for your video card. on this tab I have a button that says "Start the NVDIA Control Panel".
There is an option called Mobile, and an option on that page called Change PowerMizer Settings. There are two settings:
1) Optimize my Battery life. This will dim your display on battery. (conserves battery)
2) Not manage my power consumption. This will run your display at the same brightness as if it were on AC. (runs down your battery faster)