Diagnosis
Sporadic or dead buttons on a touchpad are typically caused by a failing membrane switch behind the panel. Moisture, grease, or just wear and tear can break the electrical connections.
How to Fix It
Start with a deep clean. 1) Unplug the microwave. 2) Make a solution of mild dish soap and water. Dampen (don't soak) a soft cloth. 3) Gently but thoroughly wipe the entire touchpad surface, paying extra attention to the non-responsive buttons. 4) Use a dry cloth to completely dry the panel. 5) Plug the unit back in and wait a minute before testing. The cleaning can sometimes restore contact.
If That Didn't Work
If specific buttons remain dead, the membrane switch assembly is likely damaged and needs replacement. This is a single unit behind the entire front panel. Replacing it is a detailed disassembly job, as you have to remove the control panel to access it.
Heads Up
Never use abrasive cleaners or scrubbing pads on the touchpad, as they can scratch the protective layer and cause permanent damage. Also, if the 'Start' button fails, you can often use the number pad (press '1' then 'Start') as a workaround for a 1-minute cook time.