@Kevin_Z I just purchased HS210 and I have the exactly same issue. I read from somewhere else my switch could be a single-pole, single-throw (SPST)? Have you resolved your issue or have any suggestion how to resolve it?

@genki  @joe43

Do you have a picture of the wiring before you installed the HS210?

@Joe43 

same problem here.

On one wall I have 1 standard SPST toggle switch to turn a bathroom fan on.  Same wall has a standard 3-way toggle switch to turn bathroom lights on. This switch is on the LOAD side of the 3-way switches.

On the other wall I have an HS210 on the LINE side.  When the fan is OFF the HS210 and the 3-way toggle swithes work fine.  When I turn the SPST Fan Switch ON and then turn the HS210 switch ON, the lights blink on/off every second or so.

I've checked the wiring connections, including the neutrals and all connections are fine.  I replace both the SPST and 3-way toggle switches ... no change

@kasaman 

I have the exact problem. I have checked the wiring several times, but still just pulses when on. I have other KASA HS210 and they dont pulse like this one. I am sending mine back and going to try another unit as I have not had this problem before. Its like the Gate of the FET is not getting correct voltage to let the transistor pass the 120VAC. But is pulsing between states Has anyone gotten any answer from KASA TP-Link as posible problem.