Have you called the US Post Office 800 number to see if they can shed any light? If you shipped Priority Mail they should have some info. I only ship Priority Mail international for this reason.
Also, some international countries require a local phone number of the addressee, might that have been the issue? Have you checked the regulations for Slovakia on the USPS web page and made sure the goods weren't prohibited?
If you can't figure out why it was returned, I would not reship it, I would refund the money including shipping and chalk it up to the cost of international business.