Here is a helpful solution for the common causes:
An oven that is too hot can cause uneven browning. Oven thermostats can change over time, requiring adjustments by the baker or calibration by a professional. To insure the correct temperature each time you bake, always use an oven thermometer. Check the oven temperature to see that it matches the temperature set on the oven dial. Adjust the oven dial up or down to correct the oven temperature.