DIAGNOSIS If the fan runs and the timer counts down but your food is undercooked, you likely have an airflow blockage or a overcrowded basket preventing proper heat circulation.
FIX First, check the physical barriers. 1) Ensure the crisper plate is in correctly and not warped. 2) Check that the rear air intake vent and the top exhaust vent are not blocked by a wall, cabinet, or other appliances. Leave several inches of space. 3) Most importantly, don't overfill the basket. For even cooking, food should be in a single layer with space between pieces. Shake the basket halfway through cooking as suggests.
IF THAT DIDN'T WORK The heating element could be failing, but that's less common. You can test by running the unit empty on, say, 400°F for 5 minutes. Carefully hold your hand near the exhaust vent at the top—you should feel a strong, hot airflow. If it's just warm or the air is weak, the heating system may be faulty.
HEADS UP Preheating is recommended for most functions. Let the unit run empty for 3 minutes to reach the target temperature before adding your food. Skipping preheat adds time to your cook.