The saying "Just do it" are just words and don't mean much in youth sports.
Words are never enough! and it's never that easy as just doing it.
You must realize realize that it is never enough to tell a player what to do or what they do wrong and then assume they can just go out and do it or correct the problem.
You can tell a player something a thousand times and their muscle memory will not let them change a habit. They need to perform things a thousand times or so in order to create a new habit and overcome the problem.
Obviously, this takes time so it is important that the coach has a great amount of patience and understands that the player is usually trying but their muscle memory will not change over night.
Also,remember, players have different personalities, attitudes, athleticism and aptitude for sports. Get to know each player and don't expect them to just do anything.