Your child should undergo a complete GI series of tests to determine if she has an obstruction or some other blockage that is contributing to this problem.
A lower GI will be able to partially determine if she might be suffering from "megacolon" or Hirschprung's Disease. If the X-rays indicate this as a possibility, then a biopsy is usually performed to confirm it.
If the tests find nothing, then she might just have a lazy bowel. If this is the case, then the easiest way to manage it may be gentle bag type cleansing enemas as needed. I don't recommend anyone going more than 48 hours without a bowel movement, no matter what the doctors may say.
Whatever the results of the tests, a side benefit is that she will get a complete cleanse of her large intestine, from rectum to cecum.