CellC - Fibre debit order not going off even though i have sufficient funds in my account

I have for the second time now discovered cell c fibre debit orders not going off even though there is sufficient funds in my account.I called Cell c to complain because I know the next thing they would wana do is to deduct 3months all in one go ...how can they breach contract by not deducting and just feel its the client's fault if their systems fail?I'm willing to cancel my contract if push comes to shove ,however I am willing to do an agreed payment plan but they refused.So I will rather cancel .