If you completed one, the rewards may take up to 3 days to be processed and added.
However, if you think that the reward should be already rewarded, first of all, check whether the offer you have already completed is still displayed on the offer wall. On the offer screen, tap on the menu in the top left corner, select Reward Status, go to Not Rewarded and check what the status of your reward is.
If there is a significant delay, please contact the provider about the offer that wasn't rewarded from their side:
Tapjoy - follow their instructions listed at http://tapjoy.force.com/
One final note: Tapjoy and the gold rewards, in general, are an external service which we can't control. While we do our best to provide an opportunity for you to obtain gold without having to spend money directly, we cannot influence the process of or speed of rewarding. Thank you for understanding.