Some fields in Email velocity template do not appear to be populated?

Hi, everyone.  I create a template below for email alert on changes to group membership:

=== Complete Event Details ===
Threat Details
Event: $introspector.getDisplayValue($event,"name")
Description:
$introspector.getDisplayValue($event,"message")

Group:$introspector.getDisplayValue($event,"deviceCustomString6")

Event time:$introspector.getDisplayValue($event,"startTime")

------------------------------------------------------------------
Source Details

Source Address:$introspector.getDisplayValue($event,"attackerAddress")

Source Host Name:$introspector.getDisplayValue($event,"attackerHostName")

Source Domain:$introspector.getDisplayValue($event,"sourceNtDomain")

Source User Name:$introspector.getDisplayValue($event,"sourceUserName")

------------------------------------------------------------------
Target Details

Target Address:$introspector.getDisplayValue($event,"targetAddress")

Target Host Name:$introspector.getDisplayValue($event,"targetHostName")

Target User Name:$introspector.getDisplayValue($event,"targetUserName")

Target Domain:$introspector.getDisplayValue($event,"targetNtDomain")
------------------------------------------------------------------
Extra Information
Base Event Count:$introspector.getDisplayValue($event,"baseEventCount")

The email is generated OK.  But apart form the name and baseEventCount, other fields are blank!

Would anyone have a solution?

Thanks.

Johnny