Debt settlement done but account does not show up as settled

I have a situation where I settled a debt with a credit card company and agreed to make 12 monthly payments. I made the payments but the statements kept coming. I didn't realize what was happening. Now, a year later I am being called collecting the new balance that I owe because I never made my payments. I have proof and the statements showed my payments. Someone screwed up and didn't mark the account as "settled". What can I do?