You ought to try and throw up a video. That would allow for a quick assessment.
Here are the issues that I see most often...
1) Insufficient height and extension on the glide.
If the glide swing does not swing up high enough, when you bring the feet to the bar, the hips move backward too much and it's pretty much impossible to make the kip or at least to make a good one. Ideally, I like for the glide to swing straight out from the bar. The "old school" method of just extending with the feet near the ground is just that - "old school." Swinging a bigger glide allows for more swing momentum and allows you to turn the kip into more of a "swing" skill.
Often, the gymnast will get loose in their butt and this will cause an early pike of the feet to the bar. Again, this will pull the hips backwards and pretty much ruin your chances of making the skill or doing a good kip if you're lucky enough to make it.
In addition, often the gymnast will rush the kipping part of the glide kip and this again causes an insufficient extension at the end of the glide. I try to get the kids to think of the skill from a 1......2-3 rhythm standpoint. The number 1 represents the long, extended glide swing. The 2-3 represents the quick snapping of the toes/feet to the bar and the kipping action itself. That part is very quick while the glide action takes a little longer to allow for optimal swing and extension.
2) Pushing vs. Pulling
The kip is a "pulling" skill through 99% of it. At the very end of the skill when you're nearly in front support, you shift the hands to finish it off.
3) Head position on the glide
Often, the gymnast lets the head stick out or lay back at the end of the glide or as the toes are coming to the bar. This arches the chest and causes all sorts of problems. Furthermore, when kids lose their grip while kipping or their hand slips off, this is often the root of the problem in my opinion in addition to probably pushing versus pulling, too. That head should stay neutral and between the arms. Throughout most of the glide...just follow the toes with your eyes and follow them as they come back into the bar.
These are probably three of the most common problems that I see - off the top of my head at the moment, anyway. Now, these things often relate to a few other problems...but, those other problems are usually symptoms of these things.