There are a lot of factors that go into why chest to bar pull ups are so hard. (Click HERE for previous discussion on length/tension relationships of muscles).
One simple reason why they are so hard to "get," is that we rarely get there. That end range of motion, from chin over to chest to bar, is a small rare place. Most of us need to spend more time there in order to get stronger there. There is much to gain from training in that range. Ask a trainer for exercises to strengthen that range, or here are a few you can experiment with.
1 - Iso holds: Jump, pull or kip to chest to bar contact position and hold for length of time 70% effort provides. Repeat 3-5 sets and build over time in volume or hold time.
2 - Eccentric or "negatives": Jump, pull or kip to chest to bar contact position and slowly lower yourself down. 3-5 sets, 3-5 reps per.
3 - Focus: Forget what the workout is when pull ups come up, focus always on height of pull.
***Very important side note. We have to make sure we do not overdo these exercises. Please consult a trainer before implementing these exercises***
Here is Collette who in her own perfect way has been working on this range of motion.
800 meter run
30 kbs (35/53lb)
30 box jumps (24/20in)