It's hard to pinpoint the exact reason without more details. Here are some possible reasons.
Different company operating the bus
The bus may not actually be run by Greyhound, rather by a partner company. It should be noted on the ticket if the bus is run by a partner. Some partner bus companies have their own tracking sites where you can track the bus, for example:
- Jefferson Lines https://www.jeffersonlines.com/bus-tracker/
- Adirondack / Pine Hill / New York Trailways https://trailwaysny.com/?vh_other_services=wheres-my-bus
Different country
Depending on what country the bus travelling in, Greyhound has different tracking sites:
- http://bustracker.greyhound.com
- Within the USA
- Between the USA and Mexico
- Within Mexico
- Between the USA and Canada (or vice versa)
- http://bustracker.greyhound.ca
- Entirely within Canada