Unpaid Debit but do have funds available

Please assist yesterday evening I had sufficient fund for all my debit orders and to my surprise this morning I get an sms to say one of the debit order has returned unpaid. I have contacted FNB via chat/email & telephone and being given a different response. My concern is the fee FNB charged me for no reason and the fact that my service will be suspended with the service provider that did not receive their funds as a result of FNB negligence