I'm not an expert but it's my understanding that most calculators use a rubber dome with a conductive part on its underside to press against a carbon coated pad to complete a circuit. My guess is it's skips because it's worn enough that the dome doesn't complete that circuit, or (best case) something worked its way underneath the dome to get in the way of the contacts.

Maybe look up a guide on how to take it apart safely to see if you can clean underneath the key and dome?