When you design a subreport and preview it, it should render correctly and as expected before adding it into a parent report. Whenever you add it into that parent report, sometimes extra blank pages will appear in between every or only some pages of the subreport.
Steps to Complete:
1. Try narrowing the subreport in the designer (not the subreport control) or moving the entirety of the report as far left as it will go
2. Once this is done, the parent report should render the subreport correctly. If you are still experiencing problems, please contact support.