COMPLAINT: My digital piano has keys that do not play, and some that play too loud/soft.
Let’s start off by explaining how the keys on your digital piano work. Beneath each key are two key contacts, one is triggered when a key is first pressed while the second determines when the key is fully depressed. Your instrument can derive the velocity of each key press by measuring the time between the activation of these two contacts.
The behavior you are explaining is most often caused by dirt and dust buildup on the surfaces of these key contacts and their corresponding circuit boards. Often the key contacts can be carefully cleaned but in some cases replacement parts must be obtained (if still available).
The first step is to try to clean the old parts. To do this the instrument must be completely disassembled so the keyboard portion can be removed. Once this is done the key contacts can be accessed, removed and inspected. If physical damage is found they must be replaced, if not they are carefully cleaned. Once cleaned everything is reassembled and tested. If there is no improvement replacement parts will be needed as the old contacts are simply worn out.