double debits

I have a personal loan with nedbank i have signed a contract and ask them telephonicly for my debit order to go off on the 29th of each month but for the last 4 months they debited on the 26th and then again on 29th this month it was taken 25th and again on the 27th and im fed up now coz then they just tell me i have to wait 7 days for my money but it doesnt take them 7 days to double debit on my account