I don't have a num pad, it's the ones above the letters
They aren't pressed down or anything, but fours and sixes would keep popping up when I typed. I can't tell if there's crumbs under them, but I've never eaten over it (but I have eaten in front of it xD).
The only thing that I can think of is that 6 is stiff and squeaky. Four is just fine D:
