Unacceptable arrangements

I have a payment arrangement going off my account for my Nedbank personal loan for the past almost 2 years. About a year and a half ago they debited my account with a huge amount of money and I laid a complaint and it took them 6 weeks to reverse the amount. Now this month they have done the same thing. I called into the loan centre where they informed me that my debit order did not go through the previous month (which by the way I don't get all my messages for my Nedbank account) so this month they have taken a huge amount off again. I can understand if they take 2 amounts off this month but not so much. So the agreement is R700 a month and now this month they took off R5566-21. Just take the 2 months off and refund the rest. I have rent to pay and school fees. This is unacceptable. No phone call from the bank no nothing. I have logged a complaint this morning Ref: ********** 4.