How do I make my sourdough more sou?

There are many variables that can get you there. First, try just extending your final fermenting in a cold retard. Try 16 hours in the fridge before baking. If you want it more sour, try 24 hours in the fridge. There are tricks that you can do to tailor your sourdough culture to produce more lactic or acetic acid. Feeding less often and keeping it in the cold of the fridge is one way. Adjusting the stiffness of the culture by adding more or less water in proportion to the flour is another way. For a fascinating look at ways to manipulate sourdough, research the Detmolder stage methods. While commonly applied to rye breads, the sourdough behavior is the same with wheat cultures. Each temperature, time duration, and hydration favor different components in a sourdough culture (yeast activity, lactic acid production, and acetic acid production).

I would try feeding the starter with some rye flour instead of only all purpose wheat flour.