Inconsistent billing

I have been with Vodacom since 2011, since i joined I have always paid my account on time and have never missed a payment. In November 2014 I started a process of switching banks. A request was sent to Vodacom and my debit order was moved to the new account. In Dec the account was not debited the whole month until after 6 January, I was not charged any late fee for the payment. End of January the FNB account was then debited again with the amount for my contract, I received a sms confirmation from the bank advising that the account was debited. I was then given a call by Vodacom advising me that my account was not paid & I made arrangements to pay it cash as they advised that the debit order had bounced. I then paid the amount in cash and was advised about a penalty amount that had been added to my account for the unpaid debit order. I advised the consultant that i spoke to that I will not pay that money they were charging me for the unpaid debit as I received a notification from my bank advising that the account was paid. It puzzles me that I have to pay a debit order fee when I did not pay that fee in Dec when they debited my account on 6 Jan.