Have you activated the card yet? - Prepaid cards usually require activation via an online portal or a phone activation process. Activation information should be on the back of your card.
Is the card expired?
Did you enter the card number correctly during checkout?
Does your card have a limit on the amount you can spend on a single transaction?
Does the card have a balance large enough to cover your purchase? - Usually you can get your balance information online or via the phone number on the back of your card.
Does the card issuer allow your card to be used for online purchases? - This information should be available on the back of your card or on the card issuer's website.
Does your card require registration? - Occasionally, prepaid cards will require you that you register them online and provide a billing address for verification purposes. You can typically find out where to do this online on the back of the card.