Firstly, please ensure your device is connected to the internet and select "More Free Coins (Tapjoy)" from the Coin Store screen. This will prompt the game to claim any pending offer rewards.

If the reward still hasn't appeared, you will need to contact Tapjoy customer support to let them know about the problem:

Very rarely, Tapjoy’s system will not correctly pass information back to the game to unlock the reward. In these cases, Tapjoy may be able to resend the reward.