Do your devices show up under Smart Home in the Alexa app? Did you try the Discover button there.
If no, then Alexa can't use them because it doesn't know about them.
My ADT Pulse hub doesn't expose any of my z-wave switches to Alexa's Discover search, so I'm stuck in a similar way.
Same symptom and cause, yours might be correctable if Wemo devices can be discovered by Alexa.