There are several possible scenarios that can trigger the bank to decline the payment:
The bank's fraud rules (which consider various factors that are not made public) have been triggered.
The bank may have placed a temporary hold on the customer's card.
The purchase session may have been locked due to multiple declined payments.
The bank requested a 3DS authentication when processing the payment that was not completed.
Resolving the issue
To resolve this issue, try the following:
Contact your bank, explaining that you are trying to process a payment with Bitstack, and ask the bank to allow all future payments.
Try processing the payment again manually in the app by tapping on the Activity tab, selecting the transaction that failed, and tapping on the Retry button. The issuing bank may have only placed a temporary hold on the card.
Remove your card as a payment method and add it again. This will authorize the use of the card with Bitstack to your bank.
Use an alternative card as a payment method.