hi Edmund,
most likely you need to go to Edit the subreport and suppress a Report Header or Footer section. when you insert a report as a subreport into another report, additional sections are added automatically.
to see what sections are causing the issue, double click on the subreport while you're in Preview mode of the main report. another scenario is that you've got blank details records coming in. in that case you can use the Suppress Blank Section option for the Details section.
-jamie