Your potatoes will stay soft instead of getting crispy if you don't shake the basket every five minutes or so. This lets the air flow around each side of the potato pieces. If your potatoes are still soft, you may also need to increase the cooking time.  Another thing to think about is cutting the potato cubes small enough. If your potato pieces are too large, they'll take much longer to cook. It's also important to not skip on the oil. While air frying is significantly less oil than deep frying or pan frying, you still need a small amount of oil to get crispy potatoes.