Can't adjust brightness on HP Compaq 6735s ubuntu 12.04

Hi, thank you for your time!

My problem started when I installed the 12.04 ubuntu version.
From the beginning I couldn't adjust brightness.
I changed the grub file (the GRUB_CMDLINE_LINUX_DEFAULT and the GRUB_CMDLINE_LINUX, and after I updated the grub), and for one or two time all was ok, but then the situation retourned the same: I can't adjoust brightness by Fn+f7 or f8, neither by terminal (with for example echo command or by setpci) neither by system's preferences.
The problem, I suppose, starts from the emptyness of the /sys/class/backlight directory.

Ask me if you need more informations, sorry for some english mistakes.
I would be very gratefull if you could help me to face this big problem!!