How much detail do the Avantix machines give about a card payment failure? Can a guard even tell if it is due to an issue with the machine / card reader as opposed to a problem with the card?
I've taken to learning what cards will decline before they even use them. Blue Natwest, solid blue Barclays, stylised world HSBC, light blue Halifax, non embossed Santander, most building society cards, solid green Lloyds all decline.
On the other hand its easy to learn which cards shouldn't decline when they do. Amex should never decline, a contactless card shouldn't decline, nor should credit cards.
The card reader tends to give a slightly different message depending on the fault;
'This card has been declined. Please use an alternative card or method of payment' - tends to be the offline card issue.
'No valid chip' - obvious, tends to be someone has put a card in backwards or a broken chip, or no chip.
'Unable to read card, please reinsert card' - this is the best message to see. 99% of the time this is a card not properly inserted or can be fixed by wiping off the chip.