I was having trouble as well. But then I noticed that it doesn't work if you have done a "Hard Lock" with the power button. If the device is left to lock itself due to inactivity it should open from smart lock. If it has been hard locked it will need to be unlocked fully again.
I'm using exchange for work. My smart lock worksSmart Lock will not work if you add an Exchange account to your phone. This is very short-sighted for Samsung - or is it Google? Meanwhile, I can use my iPhone 6S hands free without any problems using the same Exchange server configurations. This is a deal-breaker for me. I'm returning the S8 and going back to the iPhone.