A sales order is returned in the Sales Orders Pending Fulfillment Report but some of its line items do not show.
Click the Customize button and go to the Filters tab. Remove the Order Items Pending Fulfillment > Has Fulfillable Items equal to True filter. Click Preview.
If the line items show, open the corresponding item record. Under Purchasing/Inventory tab, make sure that there is enough quantity available for the location selected.