XPS 14 (L421x) Can't manually change brightness

I installed all video/power management drivers needed:

1. nVidia GeForce 630M + GeForce Expericience (up-to-date)
2. Intel HD Graphics 4000 (up-to-date)
3. Dell battery (Dell Intelligent Display ON + Dell Extended Battery Life ON + Desktop Mode ON)

When I try to change the brightness (Fn + F4/F5) the system shows the brightness slider, it moves up and down, but the brightness remains the same. The only thing that modify the system brightness is the image displayed in the notebook, in darker images, the brightness automatically goes down and in brighter images it goes up.

How can i adjust it manually?

OS: Windows 8.1 Pro (up-to-date)

XPS 14 L421x