Why are there no results using a filter on the contacts.source_hierarchy field with a value of "Import Tools"?
Data Import Wizard
Create a filter and choose the "In List" for the Operator value and then select both options "Management & Config" and "Import Tools / Contact Import" this will return the data for all cases of the import types.
In the May 2011 release the contacts.source_hierarchy field defaults changed from "32009 - Import Tools" / "8001 - Data Import" to "32001 - Management & Config" when importing contacts via the Data Import Wizard. Although the contacts.source_lvl1 has changed for these the contacts.source_lvl2 has remained the same with a value of "8001 - Data Import". Therefore, another option might be to report on contacts.source_lvl2 rather than contacts.source_lvl1 or contacts.source_hierarchy.
For more information, refer to Answer ID 4483: Modifying the search filters in a report.