A balance that continues to increase

For the last 6 months I have had a problem with the billing system in which it kept telling me that amount due by the first is zero yet I made a payment anyhow. During that time I have also paid the outstanding balance and mysteriously the next month when I got my statement it was more than the data subscription service fee even though in 2016 I cancelled all club subscription fees and account protection fees. I did not pay an instalment in November and December as in store I was told amount due was zero. I then started getting smses and calls saying that I'm in arrears and will be getting listed at credit bureau as a slow payer. I have approached customer services first calling customer care then by emailing them on 25 January 2018 asking for an audit on my account to which they rep**** with a 6 month statement; then again I emailed them 4 February 2018 after paying the outstanding balance and on the slip was an outstanding balance more than the data subscription fee and even though I sent all the details of store, time, cashier, transaction number customer services sent an email saying they will only help once I send proof of payment. They have all my details and complete access to my account how is it possible that they cannot see that I am telling the truth and that a payment was really made? Can someone please assist? Is TFG become bankrupt and are trying to recover costs by imagining costs for customers to pay? I really need some answers