Incorrect Debit order

I have a contract with telkom, my usual debit order was on the 25th,i went to change my debit order date due to new work and payday, i normally pay R359 then mnth end they decided to debit R567 when i went there they told me it's an amount that was charged because on the 25th they ddnt get the money on my account, my question was what were they doing on my bank on the 2th since we'll i changed the debit order date no1 is giving me a valid reason everytime when i call the consultants r not available, and i made the changes on the 21st of March alrdy, and chage was due in April, how possible is it that i would be charged so much money for sumthng that i alrdy reported and changed, is this part of their **** or what