Diagnosis
The membrane keypad is failing. Spills, steam, grease, or just years of use can cause the conductive contacts under the rubber buttons to wear out or become less responsive.
How to Fix It
For a single sticky button, you can try cleaning. Unplug the microwave. Use a cotton swab dipped lightly in isopropyl alcohol to clean around the edges of the problematic button. Press the button repeatedly to work the alcohol in and dissolve any grime. Let it dry completely before plugging back in.
If That Didn't Work
The entire membrane switch panel likely needs replacement. It's a single, flexible plastic sheet with printed circuits that connects to the control board. On this model, it's accessed by removing the control panel cover. It's a relatively simple swap if you can find the exact part.
Heads Up
Don't use excessive force or liquids when cleaning, as you can tear the delicate membrane or push contaminants further in. If multiple buttons fail, it's almost certainly the membrane and not individual fixes.