I need a bit more details because it's a complicated button.
The 12 function keys on the top of the keyboard usually perform their assigned task when just pressing on them. If you press the FN key, they won't perform any of their usual tasks, and will act as Fx buttons. If you checked a certain option in Settings, the opposite will happen. Check that this is true.
Most likely, the button is just bad. If you need it so badly, get a top panel replacement (which includes the keyboard). Try cleaning it first though.