A high percentage of the missed jerks I see occur from a mistake in the initial dip. Athletes sending their weight forward onto their toes, resulting in the bar also being sent forward and then missing the lift. Often this can be fixed with a simple cueing of "weight back on the ankles" On other occasions it may be necessary to have specific drills to address the problem. A favourite of mine is to slow down and pause in the dip, ensuring the weight stays over the mid-foot / ankle.

Chad Vaughan demonstrates the slow did & pause in this video;

http://youtu.be/GLh2TnGyrtw