Every single parcel.
I live in China. Yet the tracking site for USPS says my parcel had been successfully delivered, but to Atlanta, USA.
Sometime the tracking is updated before the people at the counter or computers know about it. And sometimes the tracking is not updated quick enough.
I don't know what it is like for those living in the States, but for me, when USPS tracking says that it has been "delivered" it does not mean it was delivered to me, but delivered to the district or regional post office. And sometimes just means it was delivered to Hohhot post office and still needs to make it to my city.
The only thing I can suggest, is wait another day or possibly two and call in.