Hi!  I've done this before but now I'm baffled. I've wired my smart switch as shown below (two blacks, white and green).  I did have to try several of the neutral (white) wires to find one that worked. The ones that didn't work, the light would just blink. But now I have one that supplies power to the switch and I am able to connect it to my wifi network. However, it will not turn the light on or off (manually or via the app). 
 

Help!

P.S.- ignore the three way switch next to this one. That is my next project!