When a subreport takes up more space on a page than is available on the current page, the subreport will show on the subsequent page instead of the current page.  To resolve this try the following:

1. In main report, right-click mouse over subreport box and select "Format Subreport".
2. In the Common tab uncheck the "Keep Object Together" and then select the OK button.

Now when previewed the subreport will start showing data on the current page and the overflow data from the subreport will show on the subsequent page.