Printing Multiple worksheets as a single print job

Here's a simple problem that has been bugging me for a couple years now. If I select a group of worksheets in a workbook (contiguous or not, doesn't matter) and print the group, sometimes I get a single print job and sometimes I get multiple print jobs. Multiple print jobs are an annoyance when printing to PDF or other "virtual" printers as each job requires its own filename. I then have to combine the PDF's later. I've tried everything I can think of to determine what's different about the workbooks that a single print job vs. the workbooks that create several print jobs.

Please help!

Best,
Marcus