Are you able to see what error message you are receiving from the ad server? It is possible there aren't any ads available to be called at that moment. If so, you would see the "No Ad Available" message. This would most likely indicate your code is correct.
If you are concerned there is another issue, I recommend you send a support request to firstname.lastname@example.org. Be sure to include the app ID, ad unit ID, coding language (XAML, etc...), country, and ad SDK version number.