Hi, I am sorry to say the switch buttons sit on top of a silicone membrane, which is so made as to be both switch contact (a small round piece of carbon) and button spring, often the silicone rips and or the carbon is damaged and the only way forward is to replace the contract and check the copper contacts on the circuit board for damage.
This type of work must be carried out by a qualified service engineer as much more damage can be caused in stripping down the control panel if you are not used working on this type of product.
Regards John. (ex Technics dealer and engineer)