Here are some reasons tracking might not be available:
The tracking number was just issued. It typically takes 24 to 72 hours for the information to update in the courier's system.
The first package scan may be upon arrival at a regional hub near the destination. This usually occurs when the courier's shipping volume is high and packages are processed in bulk. The courier might not acknowledge that they have received the package until it has been scanned for the first time.
The first package scan might not be until the package has been delivered.
Some shipments, like warranty replacements, might have limited tracking visibility or are not trackable at all.