Wrong bank account debited

I got finance from Wesbank in December 2015 and the debit order was supposed to go out on Saturday. I just received an sms that the debit order was returned. I have two bank accounts, Standard Bank and FNB. I always use Standard Bank for debit orders because that is were my salary goes, even with my previous car Wesbank debited Standard Bank so I don't know why now they are debiting FNB. I called and requested my icontract and in the contract it says Standard Bank. I was charged R100 by FNB for a returned debit order, I need that money back from Wesbank because I never said you must debit FNB.