Have converted Order into several invoices. Still have one more invoice to send, but cannot process another invoice against this order, it says it is fully converted.
This is likely due to the quantity on the Order being fully consumed, even though the a balance to one (or more) of the line items remains. Once all of the quantity of an order has been converted to an invoice, the order is considered fully converted, even if there is an amount remaining.
Unfortunately, issuing a credit note against one of the invoices does not return quantity to the order, and a further invoice will still not be able to be processed. A new order will need to be made to create the invoice.
To avoid the issue of running out of quantity, swap the quantity and amount fields. For example, instead of a quantity of 1 @ $1000, enter a quantity of 1000 @ $1. This will make partial invoicing simpler. This will also help avoid having to use fractional quantities when invoicing.