A debit card may decline when paying at the gas pump, even when the card is assigned to a Pocket with sufficient funds that cover the cost of a tank of gas.
Likely what has occurred is a result of how merchants like gas stations, hotels, and rental car companies, treat debit card transactions. Such merchants typically reserve a larger amount than the expected cost (say $100 at the gas pump) because they cannot predict how much available funds are in a debit account. By attempting to authorize more than the anticipated cost, the merchant is assured there will be enough funds in the account to cover the end purchase amount.
If possible, please go inside the gas station to pay at the register. Here, you can request the specific amount to charge to the Pocket and will avoid being declined.
FYI: One is not able to release holds placed on your account by merchants such as gas stations, hotels, and rental car companies etc. If the merchant does not post the transaction, the hold will be automatically released from your account once it has expired.