I have all my devices set up in SmartThings, they work perfectly. I go into the Alexa app and enable the skill for SmartThings, enter my username and password for SmartThings. It tells me I have successfully connected to the SmartThings but doesn’t give me the screen where you would Authorize Alexa. So my guess is this is where it is happening and why I can’t find any of my devices through Alexa that I have on my SmartThings hub.
Are you authorizing / discovering the devices AFTER you do the SmartThings skill enable in the Alexa mobile App?
As well, I assumed you are using ONLY ONE hub with SmartThings. Be sure if not you are selecting the correct HUB.