If you are being asked to pay again, it usually means the store cannot find your original purchase. Because your ad-free purchase is tied directly to your app store account, this typically happens for one of two reasons:
You switched device platforms (iOS ↔ Android): App purchases do not transfer between different store systems. If you bought the ad-free version on an iPhone (App Store) and switched to an Android device (Play Store)—or vice versa—the new platform will not recognize the previous purchase.
You are logged into a different store account: If you are still on the same type of device but are being asked to pay, please check that your phone is logged into the exact same Apple ID or Google Play account you used to make the original purchase.
How to fix it: Make sure you are logged into your original app store account, open your device's store, and download the ad-free version from your "Purchased" or "Manage apps" history. You will never be charged twice for downloading the app on the same store account!
If you purchased the ad-free version and are still seeing ads, we suggest you visit the article: I bought the ad-free version, why am I still seeing ads?