Nedbank swiping machine declines transactions

I called in to the merchant's department to request help with a malfunctioning swiping machine, they make me to reset and swop the sim cards in the machine then press this and that, then press this. The machine works for the first two transactions then it starts to not work again. Today on my fourth complaint Shanon ref ********** 9 told me to do the usual: remove the sim card, press this and that then swipe a transaction to see if the machine machine works. I swiped(I want this transaction not to be charged 3%) to see if the machine works and it worked., but as have been the case the third or forth transaction will not be swiped successfully.