Build Custom Report - Access Relational Data??

Build Custom Report - Access Relational Data??

I hope I'm missing something, but I have an application working great with a number of relationships between forms.  Zoho does a great job in creating the underlying default report to include the relational form for access, however when you you want to create a pivot table custom report, you are not allowed to bring in the related table (form) to use these fields for the report.

Why can I ONLY choose from forms, and not existing reports, to build another report that is truly meaningful, utilizing relationship data for reference as needed?

I'm trying to create a basic pivot report.  This report ideally should use a "class" for the column header, and this "Class" is derived through the relationship of the ItemNumber in sales_details to the ItemNumber in inventory_items