POOR SERVICE FROM NEDBANK PERSONAL LOAN DEPARTMENT

I took a loan with Nedbank in February 2019. I asked them to debit my account on the last day of the month. They did not do it, in March they debited on the 20th, in April they debited me twice on the 20th and 31th. I asked them to debit on the last day of the month because i don't get paid on the 20th. Yesterday a debit order went through again on my account. When i ask them , they say the there are two debit orders loaded on their system. I want them to stop debiting my account on the 20th, they must take their money on the last day of the month effective June 2019.