Air-fried sweet potatoes come up dry and tough?

I just tried air frying rather thickly-sliced sweet potatoes for 160°C/320F for 20 minutes. Tossed them in olive oil and sprinkled some salt before air frying too.

All the recipes I’ve read online say that the potatoes should turn out soft and moist on the inside but my sweet potato fries turned out really dry and tough. Should I be air frying for a shorter duration, or is it something else (maybe not all sweet potatoes are suited for air frying)?