There could be too many fries on one baking pan. Make sure the fries are in a single layer with a bit of space between each piece for the most crispy fries. Also, make sure that the slices are uniform, 1/4 inch slices and not sliced too thick. Make sure the slices are DRY before you toss them in cornstarch and/or oil. Wet potato slices may steam in the oven instead of getting crispy when baked.