Declined fuel purchase attempts are generally the result of swiping the card at the pump or the cashier unsuccessfully authorizing the card for an amount available on the card.
If the available card funds are less than requested by the merchant the result will be a pre-authorization hold on the funds for typically 4 days and the sale will not be authorized.
Swiping at the pump will typically result in merchants requesting a pre-authorization for approximately $200.
Cashiers are often -but not always- able to process your card for an exact amount you request. If the cashier processes an authorization for an amount greater than the available balance the transaction will be declined and the result will be a pre-authorization hold on the funds for typically 4 days.
Pre-authorization holds on funds generally expire in 4 days and the funds will be again available on your card without further action required.