Discovery refused payment of a claim

I got an hospital authoritarian approval from Discovery, during the authoritarian call, I provided 4 codes for the procedure and they authorised all 4 codes. After the procedure, I submitted an invoice from the Dr who performed the procedure and Discovery declined one of the codes, stating that the code 1807 is incidental to code 2527. Why wasn’t it picked up during the authorisation process, and only picked up when I claim. I have had to pay for the Doctor from my pocket seeing as Discovery has distanced itself. Very disappointed.