Refusing to change Debit date on my account

I have been trying to change my banking details with with my standard bank home loan for far too long and i finally send them an email and supporting documents as requested to change banking details and debit date but now they did not change the debit date, just found out that they only change banking details but not Debit date. now i just called them to change it and they are refusing to change the debit date saying its already on the pipeline and i should call them next month. so it was not my fault not to change debit date so could you please change the debit date on my account. thats all i am asking for. i have given you all the required documents you wanted.