Diagnosis
control (button) function has likely been disabled in the Kasa app's settings for that plug. It's a software lock, not a hardware failure.
How to Fix It
Re-enable the button in the app. Open the Kasa app, tap on the plug, tap the settings gear icon, and look for "Button Control" or "Local Control." Ensure it's toggled to ON or ENABLED.
If That Didn't Work
Perform a quick reset. Unplug the device from the wall for 30 seconds, then plug it back in. This can clear a temporary glitch. The side button should now work unless the software setting is still off.
Heads Up
This setting is useful for preventing accidental manual toggles, but it catches many owners off guard. If the button is disabled, a manual press will do nothing—control is app-only until you change the setting back.