Press F9 (Admin Console). Go to Lookup Tables, Help Desk, Event Policies. (The next steps will need to be taken in all event policies) Open a policy and click Configure Notifications. Click the Assigned Technician tab. Make sure the second box down is checked (Work Order modified by someone other than the Assigned Technician).
This sounds like the following issue:
The system is indeed supposed to send a "work order modified" notification to the assigned technician if a user appends additional information via e-mail reply, however, there is an issue with it at the moment.
The article contains information on how to resolve it.