Why won't keys change?

I'm having trouble playing a Neo-Geo game, Samurai Showdown 2. It works just fine, but not all the buttons are available. When I go into the menu, it says they're assigned to "None". Then, when I try to change them, nothing happens! I can change all the keys that are already assigned, but I can't assign keys to buttons that aren't assigned a key yet. I've had this problem with other games too. Does anyone know why this is and how I can fix it?

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