Non compliance with claims (PMB patient)

I went in for a procedure 23/05/2017 (properly authorized before hand). After the procedure Medshield are now refusing to pay the full account and are seemingly trying to duck and dive their responsibility. The hospital have contacted them from their side repeatedly and have repeatedly forwarded all requested documents as well as having the specialist explain the procedure to them. Every time they are contacted they either say the will phone you back, refer the matter to MSO or they are investigating. The same procedure has been paid for by the same medical aid in the same hospital without problems for other patients on more than one occasion, but I feel as if I am being discriminated against me. Now they are saying to the hospital the case is closed, and the response to me is that the claim has been submitted late. I am tired of being given the runaround and it is costing time and money to resolve this matter that should not have been an issue in the first place. I also have to continuously follow up with them as they do not respond back in most instances. All relevant documents and letters regarding claim and communication are on file with myself.