Debit Order keeps bouncing

I have a debit order from an external bank on my cheque savings account that keeps bouncing every single month while I have sufficient money in my account. I have consulted both the external bank and FNB to try get to the bottom of this continues error as it putting a dent on my bank statement if not the entire credit profile. I need FNB as my bank to step up and rectify this unsuccessfully collections that always reflects on my statement. I refuse to be told it cannot be removed as the call centre insists. I will have this debit order running from my account for the next 35 months, I cannot have the unsuccessful collection reflecting on my statement for that long. I REFUSE.