It's possible that you're required to pay for the ad-free version even if you have already paid for it. This can be because:
- You changed your device for one with a similar operating system (iOS or Android), but you are not using the same store account (App Store or Play Store). The purchase of the ad-free version is associated with your store account, so you'll need to use the same account in your new device in order to download the app without having to pay.
- You changed your device and are now using one with a different operating system. Since the stores are owned by two different companies, we can't transfer purchases from one to the other.