If while looking at your Alexa companion app, you see that a skill is enabled, but that skill's doesn't seem to work on your Echo device there could be one of two things going on:
1) The Alexa companion app and device are signed in under different Amazon.com accounts. Note that when an Echo device is purchased, it's automatically associated with the Amazon account used to purchase it, which may be different from your development credentials. To ensure that's not the case, deregister your Echo device and then set up the Echo device within the Alexa companion app.
2) The other possibility is that your Echo device and the skill in question are set to different languages. Note that Alexa considers "English US" and "English UK" to be separate languages.
To check if this is the problem, launch the companion app, tap "Settings" and then "About the Amazon Alexa App." Next to "Host Name" you'll see "alexa.amazon.com", "alexa.amazon.co.uk", "alexa.amazon.de" or "alexa.amazon.in". Each language tab within your developer account is associated with one of these. English US (.com) English UK (.co.uk), English India (.in) or German (.de).
If the language in your Alexa companion app doesn't match the language tab in your developer account for the skill in question, you will not able to use the skill on your Echo device. The simple solution is to create a new language tab within developer portal that matches the language of the companion app.
For more information, please see the related International Skill Distribution Knowledge Base article here: https://forums.developer.amazon.com/articles/45355/international-skill-distribution.html