I just wanted to note this since I never knew and haven’t seen it mentioned anywhere.
This info. is current as of May 21, 2019. My apps are fully updated (both Alexa and Smartthings).
You must disable and then re-enable the Smartthings Alexa skill in order to get routines to work with newly added Smartthings sensors.
I purchased a Smartthings Motion Sensor and GE In-wall z-wave switch and both added fine. I could control the switch from both apps and both devices read correctly, ie: can see on, off, motion or no motion and responded very quickly. I was able to set up automation in Smarthings but it would not work so I deleted them. I then set up routines under Alexa but it still would not work. The automation is simpIe, turn on when there’s motion, turn off then there’s no motion for 1m. I thought about it for a while and then figured “when there’s doubting”, log out then back in (ok, I made that up, but it’s generally true). This worked. As soon as I logged back in, my devices got re-authorized and the routines started working.
Anyway, that is my 2c. If you’ve been pulling your hair out because automations won’t work on freshly set up sensors, all you need to do is disable (log out of) and enable (log back into) the Smartthings skill in Alexa and it should work after that.