Discovery doesn't cover what they say they will.

I am utterly disappointed with discovery. I went for two of the EXACT SAME SURGERIES in June TWO WEEKS APART and they covered the first hospital stay fully and left the second stay to me. It's not a small invoice. I will not pay that. I called in July to rectify, they had told me that they would sort it out with the hospital. I have received the statement WITH INTEREST now again. I will not pay this. I don't understand how Discovery can do this?