I assume there is a printout layout and you haven't modified this layout. It sounds like you modified the data entry layout but not the printout layout. Modify the printout layout to contain all the fields that is on the data entry layout.
or modify your print button to print the data entry layout.
This sounds like the starter solution that comes with FileMaker 12. The starter solution uses a different layout for printing invoices as S Chamblee has indicated. If you print from it with the "current record" option, you will get only one line item entry. Print using "Records Being Browsed" and make sure that you are in List View for this layout.