first of all sorry for the inconvenience.
The pro version is bound to your Google Account so as long as that remains configured as the first account in your device settings, it should stay active.
In some rare occasions, the Google Play license verification may get bugged. In this case, you can force a new license check by clicking the "Buy Pro-Version" button and choosing Google Play. If the window disappears and the ads are removed, it worked. Otherwise, please follow the steps below.
1) Reboot the device and force another license check.
2) If you use more than one Google account, make sure that the purchase account is on the first position in you list of accounts. If changes are necessary, perform 1) afterwards
3) Open the app manager (e.g. "Apps", " Manage Apps", ...) in your device settings. Find the Google Play Store and press the "Clear data" button. Afterwards, open the Play Store - you should be asked to accept the terms and conditions again. Lastly, perform 1)
4) Uninstall Deliveries completely. After reinstalling, the license will be checked upon first opening it again.