The reason for this may be due to the tracking information is updated when the packages can buy the shipping carrier.
Which means the customers are not getting any tracked information of the shipment.
USPS tracking problems can occur when say the tracking number cannot be found, or there is a total outage where has gone offline.
Other issues encountered could be to do with forwarding and priority mail, deliveries of packages etc.
Usually, the tracking information of USPS will be found in the website
Even though this is a common problem but still USPS is working hard to provide the best satisfactory results for the customers.