This may be one possible reason:
With many prepaid cards, there's no owner name or billing address information nor is there a way to include that information on the card. There’s only the card number and an amount. PayPal can see the amount but, their security checks cannot confirm the card owner as there’s no name or address info. When that happens, you are asked to use another funding source.
OneVanilla recommends that for shopping online, you will need to assign a Postal Code to your Card. Simply log in to your account and visit the "Assign Postal Code" page. Here you can register your Postal Code for the first time or update your existing Postal Code. Also a good idea to ensure that you can register both your name address in your Vanilla account.
