Vodacom payment allocation

I paid my account on the 31st May and today i get messages about my account being in arrears. This is not the first time this has happened, and this affects my credit rating, which marks this as a missed payment..... Reflecting negatively against my name, and this I do not appreciate. Vodacom needs to look at their payment allocation system. If I phone, then I need to submit proof of payment, yet other months payment acknowledgement is done same day as payment.