The first reason is that the quality of the oven is not up to standard, and there is also the reason for operating it yourself. If the top is not heated evenly, it is obvious that you have not set the oven to bake at the same time, or your oven does not have that function. Generally, if you want to heat evenly, you can turn on the machine to preheat it, then put it in to bake. The temperature should not be too high. High, about 180 degrees