A kip is a tricky skill because much of it is so subtle that there's only so much a coach can do; eventually you'll do it right by accident a couple of times, remember how it feels, and be able to do it after that.
If you could describe your troubles or even better give us a video, we could probably be more helpful.
Having said that, I think the most common problem that is overlooked by coaches as gripping the bar too tight; if your wrists are staying under the bar, this is the most likely reason. When you kip, you should almost peel off the bar, then use that to regrip to the top.
I'd go back to blocks off the bar, and then glides. When you fully extend your glide, stretching long and low in the front, then try to kip. Make sure you are shifting your wrists and keeping your legs at the bar once you pull your toes up.
As already said, it would help to see a video or have more details. Kips are one of those really tricky skills to "get."