Could be an address problem. If you have even one number wrong, USPS will not deliver the package, and it will get sent somewhere else.
Print your labels on the USPS website, and they will not get lost. If you type in a wrong number or zip, they will not accept the address. You have to use a legitimate address to print the label. It will have a scan bar on the label that helps it to get to the addressee.
I have shipped several thousand packages by USPS in the past 12 years by using the USPS click and ship, and they have never lost a single one.