FNB LOAN

I am black listed against my name because FNB loan that i had and owed to them for more then 6 years, i have made monthly payments for the past 6 years but yet every even after i make payment they always claim i'm still in arrears and the amount has not changed. I recently contacted them regarding this payment as i wanted to arrange settlement and i did receive a settlement amount 15000 of which i paid to FNB the very next day, yet FNB is not willing to give me any paid up letter as i have been requesting from them. Instead customer service are just giving me problems then solutions. They say i still owe FNB even thou we agreed on a settlement. I tried to talk to the call center but could not get far as I was transferred from one call center to another. It seems like FNB does not update the info from the credit bureaus regularly. I want to know what must be done to rectify this please help me.