here's my system info.
when i press Fn + up or Fn + down. It doesnt change monitor's brightness. but on top-right corner brightness indicator moves left and right.
HOW I SOLVED IT
note: if you havent installed nvidia driver. Install it before you continue this.
this creates /etc/X11/xorg.conf . edit it.
# sudo su // become root # gedit /etc/X11/xorg.conf
find Section "device" in text. add this line
Option "RegistryDwords" "EnableBrightnessControl=1"
so it shold look like it on the rightside
from this TO this
Section "Device" | Section "Device" Identifier "Device0" | Identifier "Device0" Driver "nvidia" | Driver "nvidia" VendorName "NVIDIA Corporation" | VendorName "NVIDIA Corporation" | Option "RegistryDwords" "EnableBrightnessControl=1" EndSection | EndSection
save changes (ctrl+S).
log out. log in.
we are done!! now Fn + Up,Down is adjusting the brightness.