It's possible that the tilt table tests are being misinterpreted and/or the cardiologists/neurologists she's seen are not knowledgable in this field. It is not uncommon for patients with autonomic dysfunction to have to see many different doctors before finding one with expertise in this area. Have you considered asking for a referral to one of the major autonomic centers such as Cleveland Clinic, Mayo Clinic, etc.?
Yes, I agree with the previous comment, and that certainly has been our experience. I might also suggest looking at Medical College of Wisconsin. Our son was diagnosed with POTs/dysautonomia there. Your daughter's drop in blood pressure is not consistent with POTs, but it sounds like she may have any one of several different types of Orthostatic Intolerance issues. Medical College of Wisconsin has a department dedicated to autonomic disorders which it sounds like your daughter may have. I will also include a link to an article by Dr. Grubb, a pediatric cardiologist (Toledo, Ohio) who sees many kids with similar symptoms. It is a bit technical but I think it will give you hope that your daughter's situation is not a complete mystery. I recently tried to make an appointment with Dr. Grubb for our son, but was told he is no longer taking new patients as his wait list was very long. I didn't think to ask if his nurse practitioner was accepting new patients. I have read that others have seen him/her and have been pleased. Yes, this is definitely a frustrating experience for parents. We have seen doctors (cardiologists and neurologists) in "nationally ranked" institutions and have been frustrated, not just with the lack of knowledge but with the lack of interest. We have not tried Cleveland Clinic or Mayo. I don't think our insurance company will approve that. I'm sorry that your family is dealing with this. You are certainly not alone and I encourage you to perservere in your search for answers.