Generally pre-paid card issues can be explained by an inadequate available balance on the card.
Please be aware that most pre-paid cards will incur a charge when used for overseas purchases. You should check the small print of the card for the fee schedule and factor this into the purchase you are attempting to make.
Additionally, you may need to make your card issuer aware that you are attempting to make a purchase from overseas. This can be done by calling the number which can be usually be found on the rear of the card and you should ask the card company to activate CBP (Cross Border Payments).