Unable to make payment on account

My preloaded card is not being accepted as I am trying to make an early payment towards my account, i.e. comes back as, "Sorry! An error has occurred: transaction declined (invalid card)". I've never had a problem like this before and I have always received great service so I am quite surprised. I called my bank to confirm if my card was ok and they gave it the all good. Attempts to email, chat, call, is proving to be futile. Nobody is responding??? Very strange. If you get this message, please call me so as to see why I cannot pay towards my account.