Not adhering to my request

I requested Absa to make sure that all charges on my account are done on the 15th of every month and I was assured that this has been done. But to my shock because there was no money in my account on the last day of the month they tried to take their money, this is affecting my credit score in a bad way. Every time I try to change debit dates and changed dates with Absa Vermeulen something just goes wrong. This happened last year as well when I changed the date from 25 to last day of the month. I will not be charged for the unpaid transactions on my account by Absa because this is not my fault, otherwise I will go to a bank where my request will be taken seriously. I am unhappy with going to the bank many times as if I have nothing to do.