Hi as I state on here quite often I am in a really rural part of spain and am very lucky to even have a vet so a chiropractor is impossible but im pretty sure nothing hurts her as she will do it fine when im on theground its as soon as im on her.
As I'm newer here, I wouldn't know where you live, REGARDLESS, that is no excuse. Buy a book and video off the internet (Amazon.com is an excellent place to start) and learn about chiropractic yourself. Off the top of my head, Dr Daniel Kamen offers a pretty in depth video on chiropractic. If that's too much of a leap for your current abilities, then start by picking up a book about conformation and biomechanics and learn about the equine body, how it works and so on. Dr. Deb Bennett has an excellant 3 volume series that will tell you all you need to know about the equine skeleton and how it works. Then revisit chiropractic and tackle the situation yourself, OR educate your local vet and get them to help you.
Having said that, if she has an issue once a rider is aboard then it can only be one of two things:
1) Since groundwork translates directly to under saddle work, you aren't asking her correctly when aboard, or
2) Your added weight, or the saddle itself with your added weight is pressing on a nerve, soft tissue, whatever, and not allowing her to respond.
She isn't being obtuse just for the sake of it. She's trying to communicate with you, you only need to listen.
One is only helpless when they no longer believe in themselves.