The resistive surface inside is just dirty. You can pry the tabs at the back, open it, and wash the wiper and resistor surface with contact cleaner.
At least, you can see what is going on inside.
The resistive surface inside is just dirty. You can pry the tabs at the back, open it, and wash the wiper and resistor surface with contact cleaner.
At least, you can see what is going on inside.