I installed my HS210 switch on the first switch coming from an old fuse bock that has been since connected to a separate breaker panel. The box had the hot and neutral coming from the fuse box, 3 wire cable sent back out. I connected the white neutrals, the common, and travelers as they were attached to previous manual switch. I powered back on and made sure the manual switch still could control the lights, it did.
I noticed when that switch was on and the lights were on, nothing on the smart switch turned on. When I turned the manual switch off, the yellow/green lights came on and I began to set up my wifi. Once I had it set up, I used the app to turn the lights on, which it did. Once they were on though, the smart switch seemingly lost power and I lost the ability to control it on the app and manually. I went and turned the lights back off with the manual switch, and when I did the I heard the smart switch click on and lights came on for a second, then off for a second, then back on, back off, so on and so on. I measured the voltage at the switch and it seemed to have 120 at it when it switched on, but then dropped everything when it would switch back off.
I checked the wiring on the other switch, which had the two travelers and the common. hooked up. I then checked the outlet that it is powerin and it only had a hot and neutral going to it.
I checked the smart switch when the lights were off, and I had 120V between my neutral and hot coming in from the panel. When I switch the lights back on, the voltage drops to about 50V across all cables and I cannot get 120V anywhere, I am guessing why this is why the switch is having a hard time driving current to the lights when the other manual switch is in that condition.
Has anyone else had this problem, expecially when working with old wiring that has been half updated with newer wiring? I feel like I am missing a traveler somewhere and that hot is being sent to ground or neutral somewhere else in the circuit that I cannot see. I bought the two piece kit and have tried both switches and they both do the same thing.