windows keyboard problem

Hi everyone

I have small problem with keyboard of my MBP when running windows via bootcamp. function keys do not work sometimes. i.e. when I try to change volume or change screen brightness they don't work, however when I press [Fn]+[F5] it works as standard F5 in windows, the same with all other F keys any idea what my be wrong?

drivers for the keyboard installed in windows?

yes, installed.

for some time it was working fine only recently it started to happened. restart helps.