Likely the potato type or overcrowding.
- Use the right potatoes: For best results, use fresh, floury potatoes. Don't store them in a cold environment like a fridge. Choose potatoes suitable for frying.
- Don't overload: The quantity of ingredients in the basket is excessive. Using too many fries at once prevents proper air circulation.
- Shake during cooking: Sometimes you need to shake the fries halfway through cooking to ensure even exposure to the hot air.
Also, ensure you're not frying fresh potatoes above 180°C to minimize acrylamide formation.