There's a known issue with Google Play, where an app suddenly appears under the wrong Google account on Google Play, which causes drupe not to recognize the purchase.
The way to move the app back to the right account:
- Uninstall drupe from your device.
- Open the Google Play Store app.
- Make sure you're using the right account by tapping the Menu icon (top left corner) and checking the email address.
- If the account is not the right one, change it by tapping the small arrow next to the email address and selecting the right account.
- Close the menu.
- Search for drupe in the Google Play store.
- Download and install drupe again, and you should have your drupe Pro.