Unacceptable!!!

In February 2018,I requested Vodacom to change my debit order date from the 25th of every month to the 20th of every month been 5 days earlier. I was told that they will start the arrangement the next month (March 2018) as it was late but on that month they will debit on the 25th. I agreed to the arrangement and thought that they will debit on the 25th of February but they did not debit, I called them on the 28th February and they told me that they will debit on the 20th March 2018 double the amount of which I agreed. Now because of the above incident Vodacom refused to add a data line on my contract. I wanted the deal at Vodacom of (3MB for R129 pm for 24 months) or at Game (6 MB for R149 24 pm for 24 months) and I was declined because of there is a dispute on my account. It was not my fault that Vodacom did not debit me on the 25th February 2018 I even called to enquire about this but they went on and put my account on arrears!! The mistake was on their side!! So now why am i the being declined to add another line!!! Can I please have this fixed!!