Diagnosis
Uneven convection baking points to improper air circulation. The primary cause is incorrect rack positioning or a blocked air inlet/outlet.
How to Fix It
First, confirm the correct rack position. For most convection baking, use rack position 2 or 3. Never use the bottom position (1) for convection. Second, ensure the rear air inlet at the bottom of the cavity and the top exhaust vent are not blocked by bakeware or foil. Third, always preheat the oven to the target temperature before placing food inside.
If That Didn't Work
If rack position is correct and vents are clear, the convection fan may be operating at reduced speed or the upper heating element may have a weak section. This requires a technical diagnosis with a multimeter to test element resistance and fan voltage.
Heads Up
Do not use metal pans that cover the entire rack. They deflect airflow. Use open wire racks or ceramic/glass bakeware that allows air to circulate underneath the food.