FNB don't follow instructions - do as they please

I have a personal loan at FNB. at the end of March my previous employer didn't pay me a salary so the the loan could not be debited. Shaun phoned me on the 14th of April to arrange for a double debit to be done end April. I agreed to this without hesitation. i asked him to change the debit order date from the 25th to the last day of the month as well, as that was when my new employer would deposit my salary. He requested a letter from my new employer, which i sent. I tried to phone him to confirm this, but according to the call centre i cant talk to him. at the end of April I noticed that one full debit went off and then part of the second debit, i informed the call centre about this who ensured me the rest of the amount would be debited that very day. also she told me the debit date was STILL the 25th.. i was fully compliant with the double debit being deducted, an agreement which should be honored not only by me but the bank as well... now they taking off bits and pieces of the amount and i have no idea what is going on. I have a few months left to repay this loan and i don't want to extend it by *********** staff not deducting the amount agreed to. i need to know whats going on