Having a debit card declined can be puzzling, but there are a number of reasons why a debit card may be declined.
One of the most common reasons for declining a debit card is that the card has expired.
This will mean that the debit card is no longer being used and a replacement has been sent. The debit card expiry date will show on the card (often with either the date the card started or the issue number). This will mean that the card is no longer active. It can sometimes be the case that the card has been expired and cancelled and no replacement sent, in these cases the bank should be contacted as soon as possible.
It may be the case that the replacement has been sent not because the card has expired, but because the card user has requested that the replacement card should be sent out. This will mean that the current card will be invalidated. It is standard practice to cancel a card when a replacement card has been sent out so as to cut down on fraud. One of the reasons why people ask for a replacement card is because their card has been lost, the card is reported missing and then the card is found again. Even though the card is found it will now have been cancelled. With most banks the security procedures dictate that it can not be activated again and the user will have to apply for a new card. It can also be the case that a user wants a new design on their debit card and requests the debit card; in these cases the old debit card will also be cancelled.
Another reason why there could be a declined debit card is that there are insufficient funds in the underlying account. Debit cards do not have funds of their own and they rely on underlying accounts. If an underlying account is exhausted there may be no ability to borrow on this account (particularly if this is a savings account or an investment account). Alternatively there may be some ability to borrow, but this is set at a limit. If a transaction is going to take the card above the limit then it will be declined.
Another reason for a card to be declined is if the card is going to go over its daily spending limit. Cards can also be declined if the security has been compromised, the card has been cloned or there is thought to be some suspicion of fraud (even if this proves to be groundless).