Missing international package

My sister made a book for my birthday with messages from friends around the world. 4 months after mailing it as registered mail in the UK all that arrived here in the US was a portion of the packaging. USPS says it is the responsibility of the Royal Mail; they say it is USPS' responsibility as it clearly arrived in the US. Any ideas on how I can track it down? This package is of no value to anyone but me....