USPS isn't real-time tracking, it's just delivery confirmation. Sometimes it takes days for them to update it.
I do have a problem with them misdelivering mail all the time though... with all the mail I get that is supposed to go someplace else, I wonder how much of my own mail I'm not getting. Sometimes it's as simple as getting the neighbor's mail. Other times, its from someplace on the other side of the city... and I wonder ...?