The delivery of a video ad reward can be affected by an unstable internet connection. When there's something wrong with a connection to the internet, while the game will respond immediately to a command (such as tapping to watch a video) it may not necessarily transmit the information to our server right away, which is why you might not be able to earn the reward.
If your internet connection drops while watching the ad, this might also impact your ability to receive the ad rewards.
If this is a consistent occurance, please refer to this article for tips on how to improve your internet connection quality.