fedex/aramex international package returned by the Post Office

I ordered a package from overseas and it was due to be shipped to my PO box. The shipper used Aramex and it was handed off to FedEx to be delivered to my PO box. The shipper gave me a tracking number but it was an Aramex tracking number and not fedex so of course it didn't roll over. The package was delivered to the post office and sat there for a few weeks and then returned because I had no idea it was delivered. Now I'm trying to track the package back and nothing. The seller claims he never received a returned package? What the heck happened to it? I called fedex and of course they are useless. Anyone ever had a similar issue? What can I do to figure out what happened to it?