There could be a few reasons for this. The most common two are that either the oven temperature isn't high enough or the potatoes are overcrowded. Additional reasons could be there's not enough fat to crisp them or that the baking dish isn't allowing air to circulate well. To get crispy potatoes, make sure you first coat them in a form of fat such as olive oil. Then, make sure the potatoes aren't overly crowded and fit in a single layer on a baking sheet (not dish with sides) with enough room for air to circulate. Finally, roast them at a high temperature of 425 or 450.