Being charged interest that is not my fault

I joined Telkom mid February this year and signed up for an ADSL connection. I also signed up for a debit order and gave my banking details so that I could debit order monthly without any hassles. Come beginning of March I noticed that the debit order did not go off. Same thing happened beginning of April so I went into a Telkom store where they told me that I did not have a debit order set up and I was now in arrears with my account. Again, I gave my banking details and paid what was outstanding immediately there in store. Next month came and my debit order failed to go off my bank account again, so I revisited the Telkom store where they told me that my overdue interest was preventing my debit order from going through (although I paid all that was outstanding the month previously). Again I paid what was outstanding in the store. The next few months, my debit order has successfully gone off however I got an SMS last week saying my account could be suspended so I went into another Telkom store where they told me that again I had outstanding fees on my account (even though I know the debit order has gone off this month and twice previously I have gone into the store to settle this)