You might encounter this behavior due to an update that the application received. The developer states on the application review page from Google Play that the automatic updates should be turned off for it. If the application received an update, your purchase information is lost. However, this problem can be fixed only by the developer. In this matter, you need to contact it by the following mail address : jacksrow1980@gmail.com. To be noticed that you will need to provide a purchase proof.