My Volume "Function" button isn't working

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I have an ASUS N53SN series.Well,my "function" button works just fine with all the other hotkeys,it only has problems with the volume ones:reduce volume,increase volume and mute.I have tried anything I could think of,bu I couldn't crack it.I simply do not know why is this happening and what can I do to fix it. Have you got any idea how to fix this problem with the "fuction" volume button?