Hi,
I've just happened across this thread, and working as a Manager for Royal Mail, it piqued my interest.
Lisa, it does sound like a problem with your local delivery office/ Postman.
A couple of questions.
Do you have a regular Postman, or is it someone different most of the time? (It makes a difference, as often the "non-regulars" don't know the area, and could make a mistake).
Do you live in a new house/ estate, or somewhere possibly difficult to find?
Is it possible the item was addressed wrongly at all and when the Postman tried to deliver to the wrong house he was turned away?
If you are getting regular missed deliveries (and I know this is going to sound a bit of a pain) you could try going to your local delivery office, and asking to speak to the "Delivery Office Manager" ("DOM" - they all have one) and talking to him/her to try and resolve the problem. Most of them are only too keen to help, as occasionally it is a new/ part-time Postman that is the issue.
I'm happy to try and help in anyway I can to resolve your problems, so ask away