I've powered it off and on numerous times, still no joy. I've eliminated all mechanical options. The contacts are all good. I've changed the rubber contact strips around to see if that's where the problem lies. But it's still the same four notes that won't function. I was convinced it's an issue with the circuitry until someone suggested I should do a factory reset as that can often solve problems. It all seemed straightforward until I came up against the brick wall of no instructions as to how to do that. Any further suggestions?