Failure to debit my account

I have a problem with Standard Bank VAF.

On the 21st of November 2017, SBVAF did not debit the first instalment. I called SBVAF the following week and they said they will debit on the 1st of December 2017 and still, SBVAF did not on the 1st of December 2017.

On the 8th/9th SBVAF called me and said I'm behind with a payment. We had an argument but we resolved by saying SBVAF will debit twice on the 15th of December 2017 (they even sent me an sms to confirm this and its also on their notes). they only debited once on the 15th of December 2017.

And now SBVAF call me that I'm behind with a payment. I told SBVAF that I cant pay it now because its not my problem SBVAF failed to debit twice in November 2017 and December 2017. SBVAF say the reason they did not debit twice in December 2017 was that the system only showed in arrears with one payment not two. I don't operate their system so I cant be held responsible for standard bank system problems.

This will affect my credit score and I am not a bad customer. This is my 3rd VAF with SB and never had this issue.

I will only be able to pay outstanding payment in November 2018 when I get my bonus.

Can you please help me resolve this matter and let them remove skipped payment from my credit record because its really not my fault.