There can be several reasons:
Payment limits - This may be due to a limit you have set with your bank. Make sure that you do not exceed this limit in volume (ie the maximum amount allowed per transaction), but also with regard to the number of payments (number of transactions allowed per day).
Incorrect entry - We all make mistakes sometimes. Please check that all data such as card number, expiration, CVV / CVC code and 3DS code are entered correctly.
Browser closed - After you pay for the order, please do not close the browser yet and wait for confirmation of successful payment of the order. Otherwise, the payment may not be processed.
Back button - Once you have paid for the order, please do not click on the back button and wait for confirmation of successful payment of the order. Otherwise, the payment may not be processed.
Payment declined - in this case, we recommend contacting your bank, which will provide you with a reason why the payment was declined (insufficient funds, insufficient 3DS verification, etc.)