I did a real fast web search on "why chin ups are hard," and got a quote which is exactly what I believe, except it was said about pull-ups (same thing, really):
"Better than sixty-five percent of our musculature resides below our waistlines. Then, there is our upper body of which the overwhelming majority of muscle mass is in our backs. For many of us this is a virtually untapped and under recognized treasure of strength. While it is true that the aforementioned pulling movements do work the back, it is also true that to a great extent they lack isolation and intensity because of the heavy emphasis on the lower bodies
contribution to pulling (which does not take place on pressing movements such as military presses, bench presses and dips which, while being compound movements, are isolated from the lower body.)
The question then, is how do we increase pure upper body pulling ability, which will in turn influence the strength and power of our body as a total unit? Exercises such as bent-over row and T-bar rows, when looked at critically (especially when the trainee uses lots of weight) tend to become momentum driven, utilizing the legs to gain advantage and thus robbing the back, and our pure upper body pulling apparatus of its full potential.
The answer is simple, yet brutal. Chin ups (underhand grip) and pull ups (overhand grip), rope climbing (without using the legs) and pegboards all allow for concentration on pure pulling strength devoid of lower body influence.
Many otherwise strong individuals have missed the mark on these critical movements. Over the years I have seen countless individuals capable of deadlifting in excess of 500 lbs, struggle to do ten dead hang pull ups. Excuses abound. Pull ups (I use this term interchangeably with chin ups) are claimed to be the province of gymnasts, acrobats, people weighing under 165 lbs. and individuals with "no real lower bodies, so it is easy for them to do pull ups."
"I'm not good at pull ups because I am heavy."
"I'm not good at pull ups because I have big legs."
"I have a big deadlift, who needs to do pull-ups?"
Pull-ups are a gut check. They are unglamorous and not often an "ego lift" for strength trainers. In a lot of cases the ability to handle ones weight on pull-ups is a good indication of one?s state of body composition. It's harder to do pull-ups if you are lugging a lot of extra fat around. Of course even that is not definitive, as we shall see in a moment. The biggest reason people are bad
at pull-ups is that they don't do them."
The same applied to push-ups.
So, Kathyrn (or anyone else), I'm afraid you can't do chin-ups because you don't do them.
"It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt." Mark Twain ;-)