First, make sure that you have the newest version of the game that is available on the App Store, to see if it still happens with the newest version.
Often temporary problems with the App Store can cause issues when purchasing/restoring. Please try the following:
If you don't have an iPhone/iPad/Mac, go to step 2.
If you do, try to sign out and in of your App Store account. On iPhone/iPad/Mac, open up the "Settings" app and find "iTunes & App Store". Tap on your email there, and choose "Sign out", then sign back in again.
Power off your device completely, by holding down the power button for a few seconds and turning it off, then turning it back on.
When it's back on, open up the App Store, just to be sure it's up and running, and that it doesn't ask you to login/confirm something. Then press the "Home" button and open up the game again and try to purchase/restore.
When purchasing, sometimes the following can cause issues as well: Expired credit card, not enough funds on gift card (or gift card does not allow In-App-Purchases), or In-App-Purchases are restricted/disabled on the "Settings" app.
If it still doesn't work, please contact us
and let us know what you tried to purchase/restore.