Go to Show SQL Query in the subreport. Copy the SQL and run it in a query tool that works witwh your database, such as Toad, SQL Manager, SQL Plus, etc. See what get's returned.
If you've put the field you're trying to display in the details section of the subreport, it should show multiple values if that's what's being returned by the query. The only reason I can think of why it wouldn't be is that "Can Grow" is turned off for the subreport. In the main report, right-click on the subreport, select "Format Subreport", go to the Common tab and make sure that "Can Grow" is checked.
-Dell