chapter headers on pages

Hi:
It used to be that DocBooks that I rendered to PDF would have the  
chapter title at the top of each page.  Recently it has disappeared:  
I only see the separation line at the top.  Either there is a problem  
with page size (so that it falls out of boundaries) or with the style  
sheets.  I did not change my mark-up nor style sheets; I supposed the  
standard style sheets have changed.

How do I get header titles back?