It could be for 2 different reasons. ONE - not baking long enough - to test to see if your dark chocolate zucchini bread is finished, stick a toothpick in the middle of the bread. If it comes out clean it is finished, if not bake for another 5 minutes until so. TWO - too much moisture - you may have too many wet ingredients and the ratios are off, making it not likely to fully cook in the center. This will not be the case in this recipe though.