DIAGNOSIS You're overloading the basket. The 6QT capacity is total volume, not recommended food volume. Crowding blocks the critical air circulation path from the bottom fan.
FIX Cook in smaller, single-layer batches. There should be visible space between pieces of food. For fries or wings, fill no more than halfway up the basket. Shake or toss food more frequently—every 5-7 minutes, not just once.
IF THAT DIDN'T WORK The heating element might be cycling too aggressively at the bottom. Try lowering the temperature by 25°F and increasing the cook time slightly. The factory calibration can sometimes run hot.
HEADS UP Burning on the bottom can also be caused by accumulated grease drippings from previous cooks catching fire. Always empty and clean the crumb tray after every use to prevent this smoke and fire risk.