Inconvenience on the debit order date

I have a personal loan with FNB, we agreed that I get paid on the 20th of every moth and they will debit me accordingly. The next thing they debit me on the 18th and my salary was not paid, and I now have a return on y account. I called them this morning and they are not of help in anyway. So I now want to know what must happen? Because as I have explained to them even when I worked for them when the 20th falls on the weekend my salary was never paid on the Friday. So does this mean I will continue to have returns on my account because of them? Because I tried to explain to the lady who was assisting me and she still kept on telling me about some rdicu;ous rules.