A complete guide to create sub-reports in iReport [Jasper Reporting]: http://learnopenerp.blogspot.com/2017/11/how-to-add-subreport-in-jasper-report.html

Hi,

Hope you are using an XML data source method.

While adding subreport to main report, you need to ensure two steps done:

1. Add a new parameter to the main report with parameter class under properties, set to Object

2. While adding the subreport to main report, in the wizard select Use a JRDatasource Expression and select the parameter defined above.