We're new to the air frying machine but I do like making sweet potato fries. From raw, cutting in french fry shapes, I bakes them at 390 deg F for 15 minutes, then shake them to mix them up, then another 10 minutes. Mine are often darker, yes some burnt, on the thinner pieces and ends and the middles are done but not crispy. I eat them anyways and they are good.
Since sweet potatoes have so much moisture they rarely stay crisp. Even when I've had them fried in oil (years ago in south carolina on vacation) they tend to get soggy within a few minutes.
As an experiment you could try baking them at 300 deg F for 20 minutes, shaking them up to redistribute them and then another 10 minutes and see if that helps.