Unpaid claim

I went for a per-authorized jaw surgery procedure on 11 October 2017. 3 Days after the procedure I got a call to say that it was suddenly declined! It turns out the hospital did not supply the correct codes for the procedure for their account. Anyway, after a lot of back-and-forth, this got finally resolved.

A week later I received the anaethetist's bill and Discovery suddenly says they did not authorize it. After I sent all the documentation they requested ("clinical indication/motivation" from the surgeon), they still declined to pay it. How can they authorize a procedure and then decline to pay for it afterwards??