If I understood correctly, you have one group header with grouping by some field (Group A) and table inside detail section which reuses the report's data source and has row group by other field (Group B). We are not sure why row grouping is not working though. In the described case, the output would be each report group rendered along with repeated table (with the headers) for every row in the report's data source. If that is the case, please consider the suggested approach below.
In order to get rid of the repeating Tables, but having report's DataSource set, I can suggest to move the whole Table item to a report section that doesn't duplicate - such as group headers or footers of a static group. In this case, as in the attached report, the following approach is applied:
1) Assign the datasource of the Report itself.
) Create dynamic groupHeaderSection
grouped by =Fields.Country;
3) Add a new static group and place the table there.
) The table should be reusing parent's data source using =ReportItem.DataObject
For more information refer to Report Structure
Please review the provided sample and let us know if this works for your scenario. Consider opening a new support ticket, where you can provide your report definition, if the case is different.
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