I know that this is true on some newer laptops so it may be on older ones as well. These is often a piece of "driver" software that interacts with the OS that turns on or off the functionality of these buttons. We got in a new HP laptop a few weeks ago and I was removing all of the HP crapware that they ship these with. Somewhere along the way I deleted a piece of HP software that interacted with these buttons and none of them worked. Once I put it back all functionality came back. Could be that you have the same situation.
