Discovery claim approval -

There has been a struggle for 3 months now with an approval given on my husband's surgery, one of the procedure codes used was approved but according to discovery it is not the correct code and they are not paying for it. The doctors and I were in contact with the medical but they just keep saying they have to approve it on their side. How long does it take then? can't they sort it out and pay for it (they gave the approval ) now they have a problem with paying the code. how does it work? Doctors want to hand over the bill because it is not paid. After all, it is the fund's negligence and they have to sort it out on their part and make the payment. They do not go so far as to complete it. In over 2 years my husband has cancer this is the first time there has been approval given by the medical and then they do not want to pay out.