SQL reports launched from Reporting|Reports currently don't support sub-reports due to the way report parameters are handled.
Currently the report expression is replaced with a parameter containing the compiled sub-report.
This is misinterpreted as a parameter to be prompted for via the reporting dialog.
As it is not a simple type, it isn't rendered, and gets assigned a null value.
At report generation, this null replaces the compiled sub-report, preventing sub-report generation.
Fixed in reports, revision 5716