There are 2 different specification codes that can be used for printing 'salesperson', and perhaps you have the wrong one.

C9 is the 'default salesperson' code that is stored on the customer record. This will print that salesperson code irregardless of what has been changed on the order. If you want the salesperson code on the order/invoice, you should use the E25 code 'invoice salesperson code'. Also, E26 prints the name of the salesperson that is coming from the invoice.