Gas prepayment card

Hi all
I recently switched to Bulb after problems topping up on meter with a prepayment gas card. The same thing is happening again. I got the card and put it in the machine then topped it up and it worked fine the first time but I’ve just topped up again and the top up has failed. Is this a problem with the meter as it works some of the time! It’s very frustrating as I have a dodgy leg and the meter is on the grassy (muddy) floor outside!! Any tips on getting card to work? Thank you