There are a few possible reasons for this problem.

1 - It could happen that the payment didn't go through. You can check it in your Purchase History:

Run iTunes on your computer. 

• Click iTunes Store.
• If you're not signed in, click Sign In and enter your Apple ID and password.
• Choose Store > View Account. (If asked, enter your password and click View Account.)
• Click See All on the Account Information window under Purchase History.

Here you can see purchases you made using your Apple ID. Press the arrow to the left of the order date to see more details of your purchase. You also get an email invoice when your credit card is charged.

In case the payment didn't go through, please try to buy the game again. If the purchase is in your Purchase History, read further.

2 - It sometimes happens that due to the broken Internet connection the game remains locked after the purchase. Please use the Restore option to unlock the game:

• Run the game on your device
• Press the Buy Now button in the main menu of the game
• In the new window, tap the Already Paid button and enter your Apple ID and password. Make sure you sign in under the same Apple ID you purchased the game. The game will be unlocked.

3 - Please note that there are two different versions of one and the same game on the iTunes Store: free and paid.

• Free - Trial - You can download it before you buy it. You can buy (unlock) it from within the app.
• Paid - Full – You can't download it unless you buy it.

So, if you purchased the game and it's locked, please check what exactly version you paid for. It could happen that you purchased the Full game and downloaded the Trail version later or vice versa. Please download the version that you purchased.