DIAGNOSIS You are overloading the cooking pot. Air cannot circulate properly. The unit's performance depends entirely on airflow around the food.
FIX Use the provided air fry basket inside the pot. Do not fill it more than halfway. For best results, cook in single layers. Shake or turn food halfway through the cook time. Ensure the fan is running loudly (you should hear it).
IF THAT DIDN'T WORK The upper heating element may be failing, providing only bottom heat. Listen for a faint hum from the top rear when preheating. If silent, the top element or its relay is bad.
HEADS UP Adding oil is fine, but toss food in a bowl first. Spraying oil directly into the hot cavity can damage the non-stick coating and heating elements.