Debt order

I would like to lay a formal against Complain FNB as this debt order problem has been going on for several months I've contacted them to rectify it however it still persists.My debt date is for the 15th of every month however they keep on deducting month end then they inform me that I'm late no my bond which is affecting my credit profile and they charge me intrest on the late payment this is despite the fact that my account always have funds available.It come across that this is a new way of FNB making up revenue as I'm not the only one with this problem.