My experience is that the most common problem with a kip is gripping the bar too tightly (which results in the appearance of "pushing away" from the bar at the end). If you have the grip of death, your wrists get stuck on the bottom. If you relax your grip a bit, allow yourself to ALMOST peel as you come up, you'll be able to regrip to the top of the bar.
Aside from that, it's just making sure you have the ab strength to get your toes to the bar quickly (V-ups, toe touches), making sure you're timing it right (you should start right as your glide swing pauses at the top of the front swing), and getting a good, powerful push down on the bar.