Debit order- no assistance no reply to email!

I emailed Wesbank during beginning of March and changed my debit order date to the 1st of the month. I received a reference number. On the 26th of this month my debit order went off and bounced. I phoned Wesbank yesterday to solve the problem. The lady indicated to me that I changed my debit order date in November last year and can only change it once every 6 months. I told her that I changed jobs and didn't know about the terms and conditions when I changed it in November, I also didn't know that I was going to change jobs in November. My new pay date is only the 31st of the month. She said she can't change the debit order date and indicated that my debit order will bounce every month until it can be changed. She said that there is nothing Wesbank can do about that. She also indicated that it will reflect me as a slow payer and can impact my credit score. This is terrible, as I have always made my payments on time for 4 years with Wesbank. She was very unsympathetic and just kept asking me if there is anything else she can help me with. I cannot believe it. I am feeling very uncomfortable just accepting that they can't change my date and that my debit orders will just bounce!!