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 Google Wallet:

• Make sure you are signed in to the Google Account you purchased the game.
• Go to Google Wallet to see your transactions.

Here you can see purchases you made using your Google Account. Select the order to see your receipt and your order number. You also get a confirmation email with your order information to your Google Account when making your purchase.

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

• Free - Trial - You can download it before you buy it. You can buy (and 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.