There are a few reasons why USPS tracking might not be updating for your mail piece. First and foremost, a shipment's barcode gets scanned several times before it makes its way to its final destination.
Once a package has been scanned, it's usually pushed to the front of the queue for delivery. This means that it's usually delivered the day you expect, but there are a few situations where this may not be the case.
A mailman may have been distracted or a package has had damage to its barcode. Alternatively, it may have been sitting in a delivery van for several hours, or there might be other circumstances where the carrier didn't scan it at all.
Another reason why your shipment might not have been scanned is that it was en route but was delayed by a delivery truck issue. This happens a lot less often than it used to, but it can still happen.
This can occur if the mailman has reached the end of their shift and kept your package for a future delivery or if they've forgotten it in their van. It can also be caused by a mailman being sick or on leave.
If this is the case for your shipment, you'll need to check with your local post office for an update on your mail piece. They're the best source of information when it comes to tracking shipments that aren't being scanned by the USPS