Volume controls are potentiometers, aka “pots”. The have a slider that moves over a conductive arc. Grit can cause bad contact. You can get “pot cleaner” spray like Deoxit to clean them. Sometimes you can remove the knobs an spray in from the front, other times you need to take the case off the equipment to get the spray into the pot. DO NOT try to disassemble the pot itself. There should be YouTube videos on doing this