The reward is given when the advert reports back that it finished, and that report needs a connection. An advert watched on a weak or dropping connection can play all the way through and still not be counted.
So: check the connection is stable, try switching between Wi-Fi and mobile data, make sure the game is on its latest version, and try again once.
If it keeps happening rather than happening once, that is worth reporting - with your Player ID, your phone model and roughly when it happened.
