Locate a working point on the same line (often a test point on the back of the board or a series resistor).
Dirt can get trapped under the rubber keypad, preventing the carbon pad from making contact with the PCB. nokia 105 rm 908 keypad ways
Physical drops or internal wear can result in broken "ways" or signal lines on the PCB. Locate a working point on the same line
Moisture can cause corrosion on the copper pads, breaking the electrical connection. breaking the electrical connection.