I had this happen yesterday on an unregistered large letter. The item was sent very early October, and I hadn't received any enquiries from the buyer all that time. I contacted them yesterday and just asked them to confirm their postage address so I could check it was the same as the one provided on the order details. 

They replied quite quickly and confirmed I had the addresss exactly right, but didn't say anything else, so I have no idea why it was returned, the name was correct as well, so it wasn't something like an unrecognised addressee, could just be they changed their mind and decided to RTS instead of opening and paying for postage back...

In any case, I let them know that in cases where a correctly addressed item is refused or not collected, I can either refund the item price, or resend if the extra postage is paid, and to let me know if they have any questions or concerns. (Haven't had a reply yet - if unresponsive, I would issue the refund).