It's hard to pinpoint the exact reason without more details. Here are some possible reasons.

**Different operator**

The bus may not actually be run by Greyhound, rather by a partner company. The partners may have their own tracking sites where you can track the bus. It should be noted on the ticket if the bus is run by a partner.

- Jefferson Lines https://www.jeffersonlines.com/bus-tracker/
- 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