Once bought, drupe PRO is always yours. New device, factory reset - as long as your account stays the same, paid apps are bound to the Google Account you've purchased them with.
Do you have more than one Google account synced to your device? If so, it might be that the purchase record is on another account. 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.