If you're referring to purchases you've made in-game using Diamonds, Coins or Battle Tokens, this is the correct article. If you're referring to purchases made with your credit card, please follow this link.
If you're unable to make purchases using in-game currency, the most likely explanation is that there has been a communication failure between your device and our servers.
Since your purchase needs to be registered on the game server, if the internet fails to communicate this transaction, even though the currency cost might be locally deducted on your device, the actual purchase will not go through. In this case, as soon as the game connects with the server again, you'll see that your currencies will come back to your account.