Diamonds sent from the videos can be affected by an unstable internet connection.

When connection issues happen, the game may respond immediately to your command (like when you watch the video) but it does not transmit the information to the server right away, which is why you might not be able to earn the rewards.

Therefore, it is important to try using different internet connections and even restarting the device when this kind of issue happens. You may have also reached the limit of videos per day which could be as to why you are not getting rewards.

If this is not the case, you can only watch and earn Diamonds for 10 videos per day. Once you have reached this limit, you cannot earn any more rewards until the next day.