In our experience, this is usually because your bank has deemed the transaction suspicious or unusual.
If this is the case, your bank will usually get in touch (often via text message) to warn you about it.
Once you have identified that it's you wanting to make the transaction and you give your permission for it to take place, we recommend you wait 15-30 minutes and then try to pay again. Usually, payment will then go through successfully.
If this doesn't work, or you do not receive any such message from your bank, we recommend you try to pay with an alternative debit card.