Sales Order/s not showing under Invoice Sales Orders page
If Invoice in Advance of Fulfillment is not enabled under Setup > Accounting > Accounting Preferences > Order Management tab, then Sales Orders with Status of Pending Fulfillment will not show up on Invoice Sales Orders page.
A sales order with 2 line items, where one line has been fulfilled & invoiced while the other item has not been fulfilled at all, will not show up on the page as well. This sales order will have a Partially Fulfilled status.
Enable Invoice in Advance of Fulfillment in order to see the above sales orders on the Invoice Sales Orders page.