This is a known bug that has bedeviled FileMaker versions off and on for years. It does not occur in FileMaker 10 as I understand, but reappeared with the release of version 11.
For More Information see: FM11 printing? "Only one half of a line prints" again, like FM6
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html
Oh great. A bug that was fixed in 10 but has now reappeared in 11...lovely. Well I can't go back to 10 because 11 had some critical improvements that I can't live without. I don't suppose upgrading from 11.0v2 to 11.0v4 would help either, right? Arrgghh...I'll have to break the news to my client later this morning. And I thought I lived in a perfect world...
I'd definitely update if for no other reason that that implements the latest bug fixes.
Some people have been able to successfully work around the bug by making small adjustments to the design of their layout so that the page break doesn't fall in the middle of the text line. That's tedious and isn't likely to work if some of your layout objects slide and resize, but has solved the issue for some. (And then they switch printers and have to fix this all over again if it has a slightly different size print area...)