It can be frustrating but sometimes you just have to let things go. Some kids get it and some things come over time. The younger kids focus on the ball and forget they are covering the kid wide open under the net. We don't keep score in our division so there is still time to teach them.
GAME - Practice with one kid playing defense and another playing offense. Try to pass to the kid playing offense. He scores 1 point if he gets the pass and 1 point if the defensive player gets the ball. Over time they will learn to stay with the player they are covering.