Scenario:
User has entered time against a specific project. The User Hours Detail report returns the time entered by the user against the Project.  However the Timesheet Detail report does not return any data. It has been confirmed that the filters are set-up correctly.

Cause:
This could happen when the Select specific projects field is enabled and there are a large number of projects to be displayed in the Project Filter drop-down.
The Select specific projects section can only display the first 772 projects sorted in alphabetical order.
If you are trying to run report for a project that is not displayed in the Select specific projects section you will be unable to see data related to that project.

Resolution:
Run the report by selecting the specific client, and then the specific project. This will reduce the number of projects displayed in the Select specific projects and you should be able to locate the project that you are trying to report on.

OR

Disable the Select specific projects field and select All in the Client, Project and other sub-filters in the Project filter and then run the report.