Usually when someone lands on top of his opponent, the assumption is that there is a score. But that is not always the case. It depends on whether there is control. If there is lack of control, no score is given.
This clip is a case in point. White tries to do an ura-nage. It fails and he falls flat on his back. Blue hooks in with his left leg in a kosoto-gari type action but his hands have no control whatsoever. He just lands on top of White but he has no control. According to the IJF Referee and Sports Commission, this is no score.