Passengers advised cards do not work in the retailers?

This is usually due to the passenger trying to put through more than the balance left on the card. Check the balance using the balance checker on the Swiipr website https://swiipr.com/balance-check/. Or possibly the retailer has processed the card using the voucher button on their POS not the credit card button. Speak to the specific retailer to remind them this is a pre-paid Mastercard and confirm it was transacted as a credit card transaction