Are you sure you already finished the current level completely? If not, keep playing for a few more minutes to get your reward - the “Installed Apps” section tells you how much more time is needed per game. In other cases, it could also be that you already finished all the levels for this game. Why don’t you try out a new game?
Otherwise, you might want to refresh the game feed, restart the app or check your network connection - it could be that we have not processed all of your play-time yet. We also do not grant rewards for apps that were removed from the device and downloaded again at a later time.