This may be related to a known problem Apple has encountered in recent days with in-app purchases, which is affecting many apps. Please be sure you have updated to the latest version of the app, as we fixed this in a recent update.
If you're still having issues, there is a workaround: restart your device and open the app. Then proceed within the app as if you are purchasing the content again for the first time. We know this sounds strange, but as long as you are using the same Apple ID you used for the original purchase, Apple's system will recognize your prior purchase and you will not be charged twice.