Prepaid cards should be treated similar to regular credit cards.  You can run a declined transaction report to determine the decline reason.  

Once you determine the cause you can work with the user to see what may next steps may be.  Most prepaid credit cards have not been registered to include a billing name or address so it is possible online transactions will be declined due to the lack of information on the card.  The user can look up the credit card company that created the card, and typically go to their website and perform a registration process to add their billing address and name.  Once this information is saved they can attempt to reprocess to see if it works, or if a different decline error populates.