What happens when you sent an item with the tracking number and the buyer apparantely didnt recieved it

[mizblackberry2012]

I have sent an item with a tracking number to the buyer and he email me saying he didn't received the parcel. But i call the post and gave them the tracking number and they said they have sent it to the right address, the buyer still insisting he doesn't have it. So i email him again and said that it was sent to him and that i spoke to the post office and they said they will investigate it. He has not reply to any of my email so now im like thinking this guy is either telling lies i don't know what to do.