Teams are playing into the channels behind our full-backs all game

The first reference I’d give the back four is the relationship between pressure on the ball and the space behind the full-back. If the passer has time to clip into the channel, the centre-back needs to be ready to cover across earlier. The full-back can still engage the winger, but they cannot be surprised by the runner outside or inside them. When the centre-back travels across, the opposite centre-back and far full-back have to narrow enough to protect the centre. I’d train the back four against three or four attackers with repeated diagonal passes into the channels. Vary whether the full-back can recover first or whether the centre-back must deal with it. The question is: who can get goal-side of the channel run without opening the middle? I would avoid “centre-back always covers” as a rule. Sometimes the full-back can recover. Sometimes the centre-back goes. The rest of the line must react to whichever decision is made.