Backorder Are Not Calculated for Repeat Items
When calculating Backorder on Invoices for Line Item being partially shipped, Big Business ignores repeat items. The Backorder Quantity (B/O) is not calculated if the Item appears more than once on the Sales Order or Invoice. (5696)
Enter the correct Backorder quantity manually. Refer to the Sales Order for Open Quantities.
Related:
Listing Backorders on Invoices