Tracking numbers aren't absolute; they certainly help, especially if you pay a little bit extra for insurance, but they aren't guaranteed to stop the parcel from disappearing like you described.
Delivery companies tend to alternate when sending sending mail over long distances and that's why they can't keep tracking it. The recipient will then have to follow up with their local postal service with the tracking number (which you will have to supply) and find out where the package is from there.
Also, if an item wasn't addressed clearly, it ends up being tossed into the "undeliverable mail" bin if the return address also isn't legible. I hope this helps!
my dues paid and apologies offered, neither amend my actions, yet both ease my personal torment