Many people learning their kip make this same mistake. The fact is, that they get "impatient" and think that the sooner they pull on the bar, the easier it will be to make the kip. That is not true.
A kip has a more of a rocking motion. You do your glide swing, and you stretch your glide out fully, and bring your toes to the bar while your back is at or near horizontal around the bar. When you move back, you try to leave your toes up, and do not pull. Be patient and wait until your arms are in front of you. THEN you start to push down on the bar. Mainly, you need to lean your shoulders over the bar. I have seen so many gymnasts get all the way, but fall because they don't lean. When you lean, stay hollow, and try to keep your toes up. Also, your hands need to be over the bar. It is impossible pretty much to successfully make the kip if your hands are not over the bar. I've tried.
And remember, PERSISTENCE pays off! It will take lots of practice to make your kip consistently and with out a spot. But also, conditioning helps too. I'd really recommend climbing the rope with out legs, as it helps all muscles in your legs, stomach, and arms. The stronger you become, the easier a kip is.
Hope this helps!