Usually your bank. Sometimes a limit on larger payments, in which case a bank payment is offered instead.
Most declines are your bank
We are told a payment was refused, not why. Your bank knows the reason and can usually clear it in a moment — a new card, an unusual amount, or a first payment to a name they have not seen.
Larger payments
Above a certain amount an extra 3-D Secure step from your bank is required. If that step is not completed the payment does not go through.
On a larger payment, card may not be offered and a bank payment will be instead. That is a threshold combined with a signal — not a judgement about you — and a bank payment always works where card does not.
What not to do
Do not retry the same card several times. Each attempt is a fresh authorisation and repeated failures make your bank more cautious, not less. Try a bank payment or a different card.
If nothing works
Contact us with roughly when you tried and for what. We can see whether the attempt reached us at all, which is usually the whole answer.