Dischem card payment problem

On Sat 20th Jan I tried to pay for an item at Dischem Strubens Valley but kept getting a response of 'card not valid' yet I had used it successfully a short while before at another shop. The cashier tried other devices but all to no avail. I then paid with another card that I have which was successful. I then checked my accounts on Sun morning and was surprised to see that a transaction from the 'invalid card' was actually processed and cleared. I addressed this at the store and kept getting told that they balance and there's no discrepancy. I have subsequently sent statements of both my accounts to Pauline at the branch to prove this, and am awaiting a response, but was also advised by Pauline that I must address it with my bank. This is not acceptable as the error lies with the stores card devices. I trust that they will refund me.