Claims rejected after recieving approvals

I was approved for a femoral hernia operation. after operation I got a bill from the doctors as medical aid only has paid a portion of the account and to knowledge the bill should be paid in full by the medical aid.<br> As I queried it by the Medihelp there response to me a couple days latter was the operation can only be approved for under 18 years of age. How is this possible as got approvals before the operation and ensured that I used the approved hospital. as this has cost me +- R6000.00