When doing a lat pulldown, be sure that your torso is set to the right angle when you pull the bar down. Leaning back from the vertical means that you will be shortening the focus of the exercise to the muscles in the upper back. Target the upper chest muscles at around the height of the collar bones. Make sure that you are also in control of the eccentric portion of the lift -- the ascent of the lat pulldown bar. Intensify the exercise over time.
It's just that simple.