I did some Google searches and there were some old reports of versions of Adobe Reader not printing PDFs generated by FileMaker. I also saw one current issue on a forum. No official reports from FileMaker or widespread problems. Try upgrading or downgrading the PDF reader. Do the same with the print driver. If these suggestions don't work, standard troubleshooting techniques say isolate the problem. What is in common between these machines with the issues and/or different from the machines that work correctly.
Thanks... I did update Acrobat here locally and that did the trick. I will just include text in the email that tells customers to make sure they update Reader if they have a problem.