The most common cause of topping that I see is caused by the lower body rocking laterally off the ball.
Once the player has moved away from the ball it is hard to return the club to the proper impact position.
Try stabilizing the lower body on the back-swing by improving posture and making an effort to keep the feet and knees quiet during the back-swing. This will allow a proper shift and turn through the ball and reduce topping after some practice.