It sounds like you might have a smiliar problem to we had for a while where the postie couldnt be bothered to write a while you were out card when delivering parcels.
So he kept taking things back to the PO, and we never knew so we didnt arrange redelivery and eventually they got sent back.
Register a complaint with the PO, and insist they put a "watch" on your post, this means they will write down anything that comes through the ofice for you, so if things go missing, you can find out if they made sorting, or went missing on delivery..