Heaven help you. Did you have delivery confirmation as well as insurance? If so then they have to show you the signature where the package was signed for. A story if you will indulge me...
About six months ago I shipped a sword to California. About six weeks later the addressee contacted me and told me that the sword had never arrived. I had not purchased delivery confirmation for the sword. Therefore I got on the USPS telephone line and got the phone number of the local post office that the package was supposedly delivered from. It turns out that if a package is insured that they will keep a copy of the signing signature on file locally even if they don't put it in their system where you can get to it. The lady there was really nice and she actually faxed me a copy of the signature (probably at the risk of her job) and what do you know the signature was NOT the signature of the addressee of the package. I then talked to the USPS supervisor at that post office and pointed out that the signature did not match. He did some looking and informed me that the signature was that of the carrier. Supposedly the carrier and the addressee had an agreement worked out that the carrier would sign for the packages and leave them at the addressees secure door. Notably there was no evidence in writing of this agreement. I had kept copies of all my receipts, I had a copy of the signature etc. and provided all the phone numbers of the people I had contacted. I filled out the appropriate claim form in the expectation that I would have to wait but that I would be refunded my insurance. Instead I got a note from the postal inspector informing me that since the package had been delivered they were not going to honor my insurance. I was stunned to say the least. Yes it had been "delivered" but there was no evidence that it had been delivered to the addressee at all.
Fortunately in this case the addressee then contacted me and informed me that the package had mysteriously shown up. I then dropped the matter. I have little faith in USPS insurance at this point although I do still buy it. My suggestion is that you start making phone calls as I did. Maybe you will have better luck.