Express Scripts claims I own them over $1000 for a medication copay even after the copayment assista...

Express Scripts claims I own them over $1000 for a medication copay even after the copayment assistance program paid for the copay. I have tried multiple times to resolve the billing error and I they keep telling me I do not owe anything but it has been almost ONE YEAR now and they STILL have not clear the amount owed!