Bread-Bakers v101.n022.10

Hi Everyone!  Now it's my turn to ask a question....I've been making
sourdough bread for years from a starter that I made back in the
midwest.  It's one that we like a lot, but it's NOT as sour as a true
San Francisco bread, and no matter what I try, I can't get that
flavor.  Any suggestions?  

Thanks, Evie Werthmann