Debit order date

I have a personal loan with FNB. Now the problem is my salary date is not fixed. I work for the municipality and sometimes our salaries get paid late. I've noticed that every time there's a credit in my bank account around the 25th FNB debits the personal loan. That is not my pay date. I visited my nearest FNB branch and they phoned the head office and the gentleman who was assisting me refused to change the debit order date. I explained the situation with the municipality on pay date and told him we dont have a specific pay date. Most of the time it's last Friday of the month but more recently our salaries are late. So in order for the debit order not to return I would prefer that it's on the last day of the month or the beginning of the month. The date written on my payslip is not what happens in reality. My car is financed with ABSA and I explained my work situation and they assisted me and the debit the instalment beginning of the month. I'm trying to keep a good credit record but FNB is intentionally compromising me and I'm not happy at all about this. Worse part I have never not paid my debit orders I don't understand this hostility as I'm providing a solution to a problem beyond my control. If other banks can change dates for the benefit of both parties why can't FNB do the same?