The defensive players need to take it upon themselves to learn the schemes and assignments and do what they are supposed to do.

However, at the end of the day, it's up to the coaches to.

1. Teach the players the assignments and schemes so they can execute correctly
2. Motivate the players to work hard to master their position
3. Find and recruit the players who are capable of performing on the field.

If you argue that the players just aren't good enough and aren't doing what they have been coached to do in practice, then you could also argue that the coaches have failed in recruiting the proper players for the system and have failed in motivating their players to work hard to learn what they are supposed to be doing.