I was going crazy trying to figure out why Alexa could not see new devices that I added to my config.yaml. I have a lot of devices that work fine, but although new devices were showing up in the http://:80/api/v2/lights list, Alexa could not discover them.
The solution for me, that I finally stumbled on in the Wiki, was to use my LATEST Alexa, a 5th Gen Echo Dot, to do the discovery. All new devices discovered and, in a little while, visible at all my Alexa devices.
Fortunately, being lazy, I did not try the other convoluted suggestions that I also found! Maybe they work too in some instances.
I still don’t know what has changed in my system - the old devices used to find added Emulated Hue devices. Hope this helps someone.