Unpaid claims - numerous submissions

Paid for an ultrasound cash and have been trying for over a month to get the money paid back to me. Every time I submit the invoice there is something wrong. That error is then corrected and invoice resubmitted just for it to be rejected for another reason again. This has been going back and forth for over a month. Why they don’t give you one list of problems to fix in one communication I don’t understand. Now all emails querying is being ignored. Discovery looks for reasons not to pay. Not how they can help their clients.