Based on the decision of the United States Postal Service (USPS), from 1st October 2015, the tracking information relating to the registered letters sent to the U.S.A. will not be posted on their website anymore.
Registered International Post is not a trackable service since last year for partial countries including "United State/Canada/Australia, etc" because of the rapidly growing eCommerce business. To lower the shipping charges, some shipping method used by the merchants will provide limited tracking information.