When is a delivery not a delivery

This week I had an A-Z deliberation made against me for the non-delivery of an item. The explanation included the comment that there was no proof of either dispatch or delivery. Oddly enough, the Postage was purchased via Amazon, the dispatch confirmed by the Post Office (with receipt) the delivery secured two days after posting (and confirmed by the Royal Mail). Despite this, the 'recipient' claims a non-delivery and Amazon have awarded full reimbursement. Any ideas