Hi Gavin,
1) Get inside the Subreport > File > Page Setup > Is it set to Landscape?
2) Go to the Section Expert > Select the section that holds the Subreport > Paging tab > Make sure 'Default' is selected (Available only in CR 2008 and higher)
3) Zoom out the Main Report and make sure the Subreport is stretched wide enough so that it starts from the left most margin and ends at the right most.
-Abhilash