Accounts Department Incompetence

Good Day, I am having a huge problem with WFS. I made a payment to them in July on my personal loan. The message I got had a reference number, which I put onto my payment. A month later I got a call saying I need to pay. I told them I had paid extra and this is the reference number. They asked for POP, which I sent to them. I put my account number on and to this day that are still harassing me to pay. I have sent them my ID number as well as the loan account number. I am at my wits end with these *********** people.