The first time you go into Reporter you will need to define the Revit Categories to form the tabs in the Data Grid.
If you then Exit out of Reporter and go back into Reporter you will most likely still see the same Category Tabs in the data grid but the will not be populated as shown in below example.
To Refresh the data grid for the ALL tabs and not just the current category tab then click the Refresh All Tabs button from the ribbon which will then extract the categories data again from the current Revit Project file.
Category: Reporter