It's hard to pinpoint the exact reason without more details. Here are some possible reasons.
Broken onboard tracker
If Greyhound do not have data from tracker, for instance because it is broken, they will list the bus with this note:
We're sorry, but the expected times of arrival of this bus are currently unavailable. Scheduled times are below.
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