Absa debit unpaid

I went to absa northmead to set up a debit order to 25th for my car and the credit card,when the day come for debits to go off they didn't, only on the 18th and 31st I was debited and on these dates there was no money in my account and I have incurred unpaid debit charges, this was in july. The card division gave me a call I told them about this which was not resolved instead they did a manual debit. Again on the 21st of August absa tried to debit my account which the debit went unpaid, which means I will be charged for the bounced debit.

Please assist as I cannot be paying for something I have an agreement with them but chooses to debit whenever, is this the banks way of making money out of us with interest