Most often, you have cooked them at a lower heat, or not added enough oil. For this recipe, you want the base of the potato with the cheese to be crispy while the outside is just a little crisp and lightly browned. I recommend making sure you have enough oil in the base of the pan before adding the herbs and cheese. Additionally, you can brush more oil over the tops of the potatoes before placing them in the oven if you want them to brown a bit more.