As many users recently found out, the Windows Store is experiencing problems and unfortunately that is causing our app to believe that in-app purchases are not valid (when in fact users have made that purchase just a few seconds ago). Other app developers are also experiencing the same issues, as posted in the media: http://www.windowscentral.com/app-purchases-currently-broken-windows-store.
We have reached out to Microsoft to see what is being done on their side, but in the mean time, if you are affected by it, please private message me a screenshot of your purchase receipt and I'll entitle your account manually on our side.
Sorry for the inconvenience and thanks for your understanding.