@Targo well it sounds like that it might be a software issue instead. If you swap the switches, but then have to detach a wire to get it to work properly that seems to indicate that the software is having a problem with the three-way hook up. Maybe there's an extra setting that needs to be in place during the set up process or maybe a software update that can recognize with the configuration is.

I've reached out to technical support but I haven't heard anything back yet, and no one from the organization has commented on this problem here. I'll give them a few more days, but if they aren't going to respond to this issue, I'm not buying any more of these switches. 
 

And I'm going to recommend to my other family members who are building three new houses that they look for another product because this technology has a pretty significant flaw, and the companies technical support is non-responsive.

@steinre1 

Having the same issue.

I'm trying to use mine in a 4-way (3 light switches to control the same set of lights).  Bought the tp-link specifically because a user on Amazon said it would work - and website also recommend these for a 4-way configuration.

Having the same problem as everyone else--have to press the button on the app twice to either turn on or off the lights.  Normally this wouldn't be that big a deal to me, but it messes up Alexa and Alexa Routines will not run properly.  That's pretty much the sole reason I bought these lights--to be able to turn my basement lights off (because my kids seem unclear on the concept--first world problems I know).

Here's what I've tried:

• I updated the firmware on the light switches--didn't help
• Tried disconnecting the ground (saw someone recommend that somewhere)--didn't help
• Reversed the travel wire connections (didn't think it should matter, but I was running out of ideas)--didn't help
• Tried different locations--didn't help
   

Solution:  (At least for me in a 4-way configuration)  I added another smart switch and now it's functioning properly.  Not the solution I had hoped but, but it works, so I guess I'll take it.

@Willie92 Did you install another 3way?  I'm having the exact same issue.

@Kotoc247 Having the same issue today.

I have one that works perfectly and one that doesnt.... and before you say its wired wrong, I checked and they are wired exactly the same way.

Panel --> Mechanical Switch--> Lights --> HS210

Wiring - Mechanical Switch

Black Panel to Black Common Screw - Black Traveler Brass Screw on the same side as Black Common, Red Travler on Brass Screw Close on Green Ground Side

Wiring HS210

Black Load - Black traveler close to each other - Red traveler close to line and ground. 

Edit - so my wife has a iphone - her kasa app works all the 3 way lights like they should, I have a Android phone (Samsung S9 on Android 10) and it has a problem on two of the 3 way lights. 

@Dravas For smart 3-way switches, HS210 for instance, to work correctly in Google Home, Alexa, etc., you must have a smart switch at the power source end and at the load end.  When setting it up in Kasa, it will ask you if you are setting up 1 switch or 2 switches.  Choose 2 switches and it will go through each setup individually.   This allows the 3 way circuit to communicate the status of the lights on Google Home, Alexa, etc.  It is also how you can set up on and off times for automatic operation.

Just installed a HS210 at the 'power ' end of a 3-way circuit.  Having the same problem.  I double-checked all the wiring.  It works fine mechanically. 

I'm an electrical engineer and I believe it's a failure in the switch.  While the HS210 operates like a SPDT relay, its operation is complicated by the need to provide on/off status of the load, both to the button light and the app.  So there's a current sensing function in the switch that has failed and is confusing the logic.  

If I turn the switch on with the app, the on indicator lights and then after a second turns off again.  It also seems to depend on the position of the other mechanical 3-way switch - that is, which traveler is powered. 

I don't know the answer but I ordered another HS210 switch and I'll try it tomorrow.  It would be nice if TP-Link would monitor these rants and come up with a solution but I have little faith.

@APhelps 

Power source end and load end???

Do you understand how a 3 way switch works???

Light in between switches

Light First/Last point in wiring.

Simple diagram 

The Traveler wire is the one on the bottom.

So what happens in the app and why I think it needs 2 is that its a dumb switch and i bet it sends a communication to its pair that says hey i am off and vice versa. You would think you would have current sense that hey current is flowing through me i am on, and when another switch is hit that it senses current is no longer flowing through the switch and go hey i am off. Doing it that way you only need one switch.

@Dravas No, the 'travelers' are the 2 wires between the two 3-way switches. How the HS210 determines if the light is on or off is a mystery that TP-Link Tech Support will not explain.  But they have told me that having Travelers in excess of 50 ft. "will lead to insensitive control due to long wiring."   That tells me they are sending some low-level signals over the travelers but to where?  I can understand if there were 2 HS210 and they talk to each other.  But the HS210 communicates to the app over wifi and The Internet and there are no other smart devices in the circuit  Tech Support suggested that I install two HS210 switches on the circuit because of the excessively long travelers (70 ft.).  

@Mauly well true the travelers are both - but in residential wiring when you are talking about a traveler wire its the extra wire at the bottom.  Industrial world we just call it a 2 pole switch and move on with our lives.   So to answer your question they used to sell the 3 way switches in pairs and they want you to use it as a pair...Me being cheap understood that 2 pole switch will mechanicly work as 2 pole switch, sadly the software and the switch are not smart enough to track if its powered on with a load or not.   They decided to make the switch more complicated than it needed to be by having two switches talk to each other rather than detect load.  So using google voice it works like a charm never have to say it outloud more than once, wifes iphone works like a charm only have to hit the botton once on the phone, my android - works most of the time some times i have to hit it twice.  Oh well at $15 a switch i can live with it.