Trying to install a smart wifi switch.

Existing is a normal switch.  Coming from the wall are 2 black wires (Line/Load), a copper ground wire and neutral. 

Wifi Switch has neutral (white), 2 black wires (Line Load) and green (ground)

I connected copper from wall to green from new switch. 

I connected the neutrals (white) from the wall to the white on the new switch. 

The instructions say, line and load from the wall can go to either 2 black wires on the new switch. 

Connected everything up as the above got it all on wifi and everything.  

An hour later, I noticed lights on the same circuit breaker, were very dim.  (different rooms, but same circuit)

I took the Wifi switch out and put old one back in and all was well. 

What would cause the other room lights on same circuit breaker to go dim?