Prove Your Pay Date

In January of this year I got a personal loan from Capitec Bank. Twice now they have sent me an SMS claiming that I did not pay them on time. (As there were insufficient funds in my account.) The problem is that I get paid on the 20th of every month. This was VERY clear from the 3 payslips I submitted to them when I made the loan. I asked for a debit order on the 25th.

When I complain I get told that I must take my payslip in to prove that I get paid on the 20th and they cannot take money via debit order on the 18th (A Saturday, no less.) Fix the payment date. I simply refuse to go in with another payslip. Why was it not captured correctly in the first place? I work 6 days a week, I simply cannot run after banks doing me in because they want their money whenever they want it... Completely unhelpful, apart from putting the onus on me.