If you are able to get it to register by pressing really hard on the NUM5 key, then as @57z said, it is guaranteed to be an issue with your hardware, whether that be your laptop or whatnot.
Unless you have some fancy expensive keyboard, all of the buttons on that keyboard register as either clicked or unclicked, on/off, to the computer. It does not register "varied pressure" which you describe. That's almost like a trigger on a controller.