The common causes are: 1. The batter was not chilled enough in the fridge before baking. 2. The butter and flour were not coated evenly on a pan. 3. The pan was left longer at room temperature while/after piping the batter. [Image below] With this batch, the pan was sitting at room temperature much longer. You can see that the caramelization is not even here and there. - Try piping quickly and immediately put it in the oven to create even, beautiful caramelization.