What are you switching on and off? Are you turning it off from the Kasa app, an Echo, or the button on the Smartplug? The light should be bright green when the plug is On, and dim green when the plug is Off. When you press the On/Off button on the plug, do you hear a click such as you hear from the Smartplug that functions correctly?

In the meantime, I fixted the problem. It seems, that my HS110 has an wobble contact. After hitting ist some times, it were able toll switch.For clarification (Addition to problem description):The light did change, it made a clicky noise. But it still delivered power. The problem is NOT dependent on the used switching technique. Alexa, by hand or with the app nothing worked!