Wesbank....bad experience with change of account

I changed my bank account with Wesbank on 9 December 2019 for the 31st. I by myself saw that they did not debit me and I called them on 2nd to ask why. I was told the debit order was success. I told them that it's not possible because that account is closed. Only then did they process the bank account change. I had to call again on the 3rd to advise them to debit me again on the new account. Even then I had to insist because 'the debit order did not return'. What I do not appreciate is the message they sent me today that my failure to pay will affect my credit profile. Whose fault is all this and why must I be the one who suffers the co sequences of your actions ?