Accumulating debt charges- Ineffective payment

FNB will shortfall your account that is a “Student Account” to a large amount of money and say that the payment has not been in “effect”. The agents are rude and if you do not comply with them on the online services they switch off the board “act disconnected” to avoid dealing further with your finance matter bare in mind this is a bank dealing with your money, a very sensitive matter and they just shut you out selfishly, the first agent “Lemogang” did not even bother to assist me, second agent “Etienne” said that the reason why I have a debt of more the R400 is because a transaction that occurred on the 29th of December was ineffective, bare in mind the messages prove that money was pulled from my account and Im saying there is proof, she disconnected herself , the balance on my bank is stil -R459. So you are telling me, on the 23rd pf January 2025 (Januworry) a bank can decide to add a debt of December (Festive) allegedly as if I did an overdraft??? I paid the amounth this year only its inneffective