Not paying service providers

I had a procedure done in November 2016, and I had obtained pre-authorisation prior to the procedure. In January 2017 they did not pay the doctor quoting "waiting for authorisation", which they had. Since then it was an uphill battle as they sent the account to the stale department who had to review it and review it and review it. They told me in October last year it would be paid in December. They still have not paid the doctor for services rendered. I don't know what to do anymore as they had no valid reason for not paying the doctor, it seems that incompetence is rife at this medical aid.