Debit order debited on a date not agreed upon.

I have 2 policies with Old Mutual, One is a life cover for myself and the other is a policy for my parents. I instructed Old Mutual to collect the debit order strictly on the 20th irrespective of the day of the week because I only get paid on that date, not before nor after. I asked them to debit me the following working day should the 20th fall on a Sunday or Saturday and not Friday but they keep debiting me before the 20th. Now the problem is that I am using a cheque account and each time the debit order bounces it records the insufficient funds and it accumulates charges on my account and thereby giving me a bad record on my account because the bank takes it that I am unable to maintain my account and that could lead to me being unable to qualify for other facilities with the bank.What I don't understand is that this only happens in one policy, which is the one for my parents. On my life cover it does not happen like that, if the 20th is on a weekend I get debited on the following Monday. I need that to be fixed or else I have to cancel my policy with them.