Filtering outstanding invoices by project

Each of our sales customers is allocated to a specific project.
We import sales invoices using the Clearbooks import routine
When we view the sales invoice details the relevant project is shown on screen - however, when we try and list outstanding invoices for any particular project it excludes any ones that were created via the import facility - the only ones included are any that have been amended subsequent to the import.
So it seems as that although the project details are visible on any invoice they are not being correctly recognised when trying to list outstanding invoices by project.