This may be a silly question, but are you posting these in the same mail box? A few years back we sent a letter that never arrived, and then our neighbour had the same thing with a few letters, seems we posted in the same mailbox. We posted elsewhere after that and no more problems. We were later told by a friend who drove by this mailbox daily, that one on occasion they saw a postal worker with a bag of mail on the ground and he was rifling through it. These days I only post letters at the Post Office.
