is it set to list view? or are you using a portal?
Do multiple lines mean that you have multiple records? Do you use a line items table to list the items for a given invoice record?
If not, you may want to examine this demo file created by Comment on another FM forum: http://fmforums.com/forum/showpost.php?post/309136/
could be a portal , section in invoices drops down when a new line is added , there is nothing in the inspector to indicate. Double clicking the grey area brings a "Portal setup"window. asking :
sort portal records
filter portal records
Sound like you have a portal. You might want to check the filter to see if that is keeping the other line items from showing, also make sure the portal is set to show more than 1 line. If those don't fix it you'll have to dig into the relationships
See the demo file I recommended for a way to use a portal for data entry but to use a different layout without a portal for printing the invoice. This is a much more flexible way to print your invoice.