You would first want to investigate how the report is built, and this is done in Advanced Report Creator. This program is password protected and you need to have completed the relevant training to have access to the program.
Open the report you want to investigate and look in the Fields tab.
The Tables and joins in this report section in the top-right of the window show the tables used in the report and the order they will be searched.
There is a button to the right, View/Amend Table Links, that brings up a new window detailing the link text; and how the table is linked to the previous table in the list (the first table should therefore be blank).
The Selection tests section shows what data is to be included in the report.
The Show selection tests for dropdown allow you to switch between the tables used in the report, as well as calculated fields. They will show what criteria the report is using to determine what should be included.