There are several possible reasons. When we submit a payment authorisation to your bank, they have automated systems that determine whether or not to accept the charge. These systems takes into consideration various signals, such as your spending patterns, account balance & funds availability, card information like expiry date, CVV, billing information etc.
These signals are constantly changing, so a previously successful card might be declined in the future. Even if all of the card information is correct, and you have previously had a successful payment with this card, a new payment authorisation can still be declined by your bank’s fraud systems. Please contact your bank to understand as to why your payment was declined.
In cases where your payment is declined, you may use alternate cards / payment options as made available to you at the checkout page.