I think these Alexa commands will help you. If you would not able to solve this issue then you should contact cutomer support for Alexa App. In these days lot of peoples are using alexa app. It is the best app for amazon echo devices. Many functions and commands in Alexa but we don't know about it. So, we should have good knowledge about it, Because in upcoming years many Amazon or other companies products are coming.
I had the same problem, but found it was how I was saying it. I tried saying "Alexa play my workout mix playlist" and Alexa couldn't find it - even though Spotify was my default music player in the Alexa app. However I just tried "Alexa open spotify and play my workout mix" and she picked it right up. Hope that helps!