Unfortunately, that's a question that only the United States Postal Service can answer. It could be delayed for some reason, perhaps the person was not home, perhaps they have mail being forwarded, or perhaps they are away on vacation.
The USPS Help Line is 800-275-8777. If tracking has stopped, we suggest you contact the Post Office closest to the delivery point and ask to speak to the Post Master. You may also visit the official USPS website www.usps.com. to check for any immediate updates. Updated delivery information will not occur until the USPS letter carrier has updated the status of your letter or package.
If it has been more than 3 weeks and you see no updated delivery information we suggest calling the LOCAL USPS Post Office responsible for the letter's final delivery. The Post Office closest to the final delivery address Zip Code should be able to research then track why the package was not properly tracked or delivered.
To locate the closest USPS Post Office responsible for mail delivery call 800-275-8777 and be ready to provide them the Zip Code of the final letter's delivery address.