It could be that you have an incorrect email address on the item.
Click on More actions to the right of the item and then Sell similar. Scroll down and check the email address to make sure you have not made an error.
If it is a viable email address add it to your paypal account.YOU can add mo re than one.
If not the buyer needs to reclaim the payment and pay you again to the correct email address. They can do this through paypal pay for Goods and send money' tab in PayPal, make sure they select pay for goods' and enter the item number and seller ID.
Alternatley you can use the 'request money' tab to send an invoice, selecting 'goods' and enter the item number. Don't select or accept a 'personal' payment or you will have no Seller Protection.
If this does not solve your problem come back again as there are others who will help.