If you uninstall and reinstall the game, or change your phone, the game may no longer recognise your purchase of the premium version. 

On Android

Normally, purchases are restored automatically as soon as you are connected to your Google Play account. If this is not the case, go to the Google Play Games app settings. You should find a "Change account for games section". In this section, there is an option to "Change account by game" in which you should be able to select the game, and it will open a window.

If the window indicates "Signed out" under the title of the game, this might be the problem. You can then select the address with which you used to be signed in and the synchronization should work again.

On iOS

Check that you are logged in to your Game Center account, then click on the "Restore in-app purchases" button, located on the pop-up that offers you to purchase the premium version.