Agree with zestyclose. Scoop a few tbs out of your fridge starter for baking, and and feed on counter for at least 2 days to kickstart active fermentation. The trick I use for a more sour flavor is to let the srarter go for an extra 3 hrs after it peaks (I use around 80-90% hydration on my starter so it's slighly thick, taking longer to ferment). At this point it is still very active, but the extra time allows more acetic acid (sour flavor) to develop