DIAGNOSIS This is almost always a crowding issue. You're probably overloading the basket, which blocks the rapid air circulation that's essential for crisping.
FIX 1) For best results, cook in multiple smaller batches. 2) Make sure your food is completely dry (pat potatoes or tofu dry). 3) Toss items in just a teaspoon or two of oil—too much will steam them. 4) Shake the basket vigorously halfway through, don't just give it a gentle stir.
IF THAT DIDN'T WORK Check that you're not using the "reheat" or "keep warm" function by mistake. For fries, use the "Air Fry" function, set to at least 400°F, and always preheat the unit until it beeps.
HEADS UP The non-stick coating on the basket can actually hinder crisping if you don't use a little oil. A light coating helps achieve that golden texture.