The vents in the oven allow hot air to circulate. If the vents are blocked by food or other debris, which can prevent air from circulating. Check the vents to ensure they are clear and not blocked by food or other debris.
The control board manages all of the functions of the oven, including the fan oven mode. If the control board is damaged, it can prevent the fan from working properly, even if the fan element and thermostat are functioning correctly.
The thermostat regulates the temperature in the oven, including during fan oven mode. If the thermostat is faulty, it can cause the fan to turn off or not work properly, resulting in uneven cooking or baking.