Not updating their account in my name

I have an account with Russells for that I am paying for over a period of 12 months. I have never missed a payment on my installments. In January the company debited me as normal. A week later I received an sms saying my account was overdue. I called the number that came with the SMS and the phone was not answered. I even emailed the company asking them to update my account. This has not happened. Now they have charged me an interest and say I owe them the January installment which I have proof that it was paid. But they have not responded or acknowledged the emails with me requesting them to update my account. I am ****ed off and taking this further because this is prejudical to my name.