problem with sourdough starter
I followed Mark Bittman's recipe to create my own sourdough starter, and while the resulting bread has a nice thick chewy crust, chewy interior and the right texture, it's not very sour. In fact, it's not sour at all. It does taste quite delicious, but it isn't sour.
The starter was made with active dry yeast, which was on hand, proofed to activate it and mixed up according to the recipe. It sat half covered on the kitchen counter for 3 days, stirred religiously every 8 - 12 hours and did its bubbly, frothy thing.
Used unbleached AP flour and filtered tap water (Brita pitcher). What can I do to make my starter more sour or should I just try to make a new one?
PS. I'm a novice bread baker. This is the first time I've turned out bread worthy of eating. All other loaves have been turned to croutons (eating lots of salads!) or donated to the local ducks!