There are a couple of things you can try to address this. First double check you are using the correct Apple ID. Sometimes Apple ID’s can be mixed up on shared devices or within families or workplaces where people share an account. You can check this by opening the App Store on your iPad and at the bottom left of this page, your Apple ID will be shown. Alternatively open Settings on your iPad, then tap iTunes & App Stores and your Apple ID will be shown here.
If the ID details are correct then check to see if there are any restrictions on In-App Purchases on your iPad. Go to Settings > General > Restrictions > Allowed Content > In-App Purchases and make sure this toggle is switched to ON.
Once these settings are correct, you should be able to restore your purchases by tapping the Restore Purchases button inside the app. The Restore Purchases option is located in the top right hand in the menu options. Enter your Apple ID and password here when prompted to log in and the application should then renew any purchases available under your subscription and offer them for a free download in the Library screen.