Experience with Aflac not paying on cancer policy?

My father has been going through cancer treatments for 3 years. Initially Aflac honored his policy and paid on the first year or so claims. Now his claim amounts are substantial and we've hit wall after wall with no response from Aflac. When we call it's a variety of excuses (codes missing, his name missing on papers, etc) and requests to resubmit. Any idea how we escalate this? Or who we escalate this to?