@Justaguy. Have you been able to figure out what's happening? I have the same issue. If I switch off from "main" switch, then the other one is dead... I have to turn on the main switch in order for the 2nd one to have power... very weird...

@TechGrl I have had 2 out of 7 work.  Frustrating.  They work once or twice then stop.  It seems that they are connected correctly since they work briefly.  

@gteagle I had a similar issue and it took forever to figure it out. Here's my setup: 

I have two three-way switches installed on a staircase that controls a series of lights; two upstairs. Here is a photo of the swtich upstairs. 

This upstairs switch is connected to the downstairs switch and works. At long last, it works. Now I upgrade to the HS210. This one did not work at first. 

When I tried to connect it all the same way, the light switched on and off, constantly. I fixed that by switching the traveler wires on the downstairs switch, which I had left alone as the generic 3-way. The light came on, but as soon as I turned it off, it would not come on anymore. The app clicked, the switch clicked, but no lights. 

I went to the HS210 and verified the wiring. I had my black and red wires connected which go to the downstairs switch. My white power line (yes it's white in my house) was connected to the black screw for power. The ground was connected via cap to the copper grounding wire. The neutral wire, coming off the switch, I had connected to the wire and black capped wires coming from the light, not realizing that wasn't neutral. On a whim, I connected the neutral wire to the grounding copper and wire and IT WORKED. 

Now it can turn on and off via the switch, or the app. I drew some terrible diagrams. You can't really see it, but I denote that the ground wire, and the white neutral wire coming off the switch should be paired, along with the green ground wire off the switch. Inside the recepticle, there is a white and black wire capped, presumably for the light. You can't see the white line but I denoted the word combine there. 

@NickOuim 

what screws? Screws to the wall or screws holding wires?  

@Justaguy. 
have you found any solution to this because I'm having the same problem?

  @gteagle I'm have the exact same issue . One switch works fine and the other one does not turn the lights on. Were you able to find a solution ?

  @Switch_user no i haven't been able to find the solution. I think it is a defective product. Sometimes it works sometimes it doesn't. It starts working once the switch is reset in the app.