Note: Rewarded ads are in some cases referred to as incentivized ads; both terms always refer to the same kind of ad. In the SDK code, we use the term 'incentivized'.
Make sure you've set incentivized to true in your playAd options.
Check that you're passing a username to playAd, which will come back in the %user% param of the callback.
Lastly, test your callback to ensure it's a valid endpoint.