Data displayed by nested data is not accessible on upper levels - Data Scope
Instead you can have data on the upper level, and then bind the nested Table item to the container's data object via expression - Use DataObject as a datasource for nested data items (Table, List, Crosstab, Graph)
You can also use a data model that exposes properties with custom getters and collection properties which you can use as data source for nested Data items and calculations in the report - How to Databind to Collection Properties
I hope this helps.
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