Sometimes a purchase needs to be restored, such as when you get a new phone or reinstall an app. To restore a purchase you made previously, go to Settings/Menu on the main screen of the app, then tap "Restore Purchases".
The same screen with buttons to purchase Premium Pass will also have a link to "Restore Purchases" as well.
If no previous purchases are found, make sure you are signed into the same Apple account you made the purchase with. You can check this in your Settings app.