Our defenders stop when a midfielder runs into the box

I’d coach the space first and the runner second. When a midfielder arrives late, the back four are often already occupied by the front line. That means somebody in front of the defence has to recognise the runner or, at minimum, communicate that the responsibility is changing. The holding midfielder is important here. They do not have to follow every runner into the six-yard box, but they should be able to see the ball and the players arriving from deeper positions. I’d use a box-defending practice with a back four plus two midfielders against a front five. Start the attacks from different wide and central positions so the late runner changes each time. The cue is: who can see both the ball and the runner? At U14 I would avoid assigning permanent man-marks. The players need to learn that responsibility is passed between units as opponents move through different spaces.