You should be using the print layout based on the line items table. Open this layout and then go to Layouts menu > Part Setup then double-click the Body part and make sure that "page break before each occurence" is NOT checked.
1) From Invoices side - Place the Invoice information in leading part with the portal in the body. Create as many portal rows as you will EVER need - keep expanding the body to allow for them. The Header, Body and Footer must total less than 5,000 px total. You can usually fit approx. 400 portal rows. Then select both the portal and any fields inside and select Format > Set Sliding/Printing and Sliding up Based Upon. Also check 'also reduce size of enclosing part.'
2) Print from LineItems - each body row will be a lineitem (skip using a portal). Create a leading part (based upon your LineItems Invoice ID). Sort by the LineItems InvoiceID (critical). Then go to Preview mode.
If only the first portal row shows, I suspect that you have the fields within the portal either too high or too far to the left. Use your Object Info (or Inspector) to see how high (and to the left) on the layout that your portal sits. Then make sure that your fields are not even 1 px further left or high.
Thanks Sunmoonstar_13 and LaRetta. It's working now. I had included the fields, but not as a portal on the layout. Once I recreated the portal everything worked fine.
LaRetta, thanks for the extra tip on sliding/printing. The receipt is adjusting as it should based on the number of items purchased.
Hey Nick, it is a fallacy that one should always print from the LineItems table. It was started by those who have no idea that portals can reduce. Invoices and their lineitems is good example . There are also times when it is REQUIRED to print from a parent layout because there is a mix of left-outer joins and a summarized report with several parts and a portal of related records (placed in the body) is required. And yes, it is usually easier to print from LineItems and that's why I provided both perspectives.
It is always a good idea to never say never or always. :^)