I have hooked up HS210 to line with travelers, neutral and groun (line to black, travelers to top screws (per instructions).  Other 3- way (load to light) dumb 3-way switch.  Switches work great manually from both ends.  App will show proper state when switched manially from HS210 (takes a while for status to change in app).  When in the app, will turn on light, but takes 2 presses.to turn off in app.  Then sometimes the light indicator in the app is wrong (shows on when it is off or off when it is on).

Do you have HS210 on both 3-Way locations for the app to work correctly?