Help

Hello I recently sent a parcel to the North from an post with Registered post and Tracking and on the 4th day i checked tracking number which states delivery has been made all good i thought untill today when the person who bought item from me in north says they have not recieved the item?
I have contacted an post in dublin and they say they will contact back with info tomorrow however how can a signed upon delivery item which has been marked down for delivery not have arrived? I asked name of person who signed for delivery along with northern carriour who is supposed to make delivery and awaiting info
Question, Would i be correct in believing i have met my obligations for delivery which was confirmed by delivery tracking info and not liable for repaying cost of item or if item has been stolen am i still expected to return amount paid even with proof of Registered sign for delivery along with delivery confirmation from an post.