They may be overcrowded in the basket, cut too large, have too much moisture, or too much oil has been used. Sweet potatoes in general are not super crispy potatoes like russets or Idaho potatoes. However, using the tips above along with adding the arrowroot or cornstarch, leaving the skin on, and doing the optional soak will help the sweet potatoes to be as crispy as possible. You can also blot the potatoes with a paper towel after cutting to soak up much of the moisture.