@kirbinster 

I have the same issue, the switch works fine on the physical button, But in the Kasa app, it always shows as OFF after a few seconds. No matter I install the HS210 to the power end or the load end. Very disappointing experience. Now because of this, I can't use it in my home automation system. I have to manually check the light status through a wifi camera to make sure the switch is doing right. 

  @HandyFrank 

I was able to overcome the issue by resetting the switch with the small button on it and ensuring the smart switch resides in the location that has the wire going out to the fixture (if only one smart switch on that circuit). Reset every smart switch on the circuit with the button and configuring the switch with the wire going to the fixture first (if more than one smart switch on the circuit).

  @kirbinster 

kirbinster wrote

Called support and they had me delete the Kasa app on my iPhone and reinstall it. Once I did all is fine in both the app AND in the Alexa app. Very strange, but it fixed things.

Thank you so much for this information.  I had the same problem and tried your solution and it worked!