Please test to reconfigure the SubReport item's reportSource by selecting the 'Type name' option to specify the report. Then double-check if all reports' connection strings are copied in the viewer's project configuration file. Test also if any data for sub reports is retrieved.
in case of using the old ASP.NEt WebForms ReportViewer and out-proc session state mode settings, please check Design Considerations for Out-proc Session State.
in order to provide you more accurate suggestions, please open a support ticket and send us a demo project that reproduces the problem.
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