Most issues associated with using pre-paid cards are explained by an insufficient funds balance on the card.
This may be caused by the fact that many pre-paid cards make a charge when used for international transactions. You should check the small print of the card for the fee schedule or talk to the card issuer and factor these fees into the purchase you are attempting to make.
You may need to advise the card issuer that you wish to make an international purchase and request that they activate Cross Border Payments. This can be done by calling the number which can be usually be found on the rear of the card.