Make sure that you sort the records with a sort order that includes the "when sorted by" fields you've specified for each of your sub summary parts.
Are you talking about the "Sub-summary when sorted by" option?
In this instance I selected the Person_Fullname & MeetingGroup fields as sort and summary fields when I created the report.
Apart from the "Part setup ..." where do you specify sorting options?
Pull down the records menu and select Sort
Specify the same field in this sort dialog that you did in the "when sorted by" section of the sub summary part setup.
Got it now - thanks
That works when I'm in native FM.
However, when I use a script to open the reports I get the original result - the sub parts disappear.
I'm using the following script to open various reports:
Allow User Abort [Off]
Go to Layout ["Groups Report" (tGroupPerson)]
View As [View as Form]
Enter Preview Mode 
Print Setup [Restore; No dialog]
Pause/Resume Script [Indefinitely]
Perform Script ["Go To Previous Layout"]
Right after your Go To Layout step, include a sort step which holds your sort by leading part.