Package returned from the UK

I have another package returned to me that the customer says they never received. There was duty owed on the package. Royal Mail has a sticker saying the item "not called for" for over a month. I know I told this person to check with her Customs Office. I've emailed HM Revenue & Customs to explain to me how the process works so maybe I can direct the next person that tells me they never received a package. If anyone has info on this subject please email me at cindy@resinobsession.com