Can't get my debit order date right

I have a new contract with Vodacom and since it began a few months ago Vodacom has never managed to debit my account on the date that we agreed on. I have called several times to ask them to rectify this to no avail. The only time they got it right was August of this year. This month we are back to the old story again. It is very inconvenient for me to have to remind them each month when is my debit order date. My bank is charging me for the unpaid debit and Vodacom also charges me a late payment penalty fee of R100. This is very unfair. I won't threaten to move to another service provider because I love Vodacom. This is very frustrating for me. Can somebody please help. My debit order date is the 20th of each month.