Why would a sweet potato still be somewhat hard after being cooked for one hour at 400°?

This is what I do: I always pierce the skin of each sweet potato with a sharp knife and then wrap in aluminum foil. Then place the potatoes on baking sheet (as they can leak) and cook at 425°F for about 1 1/2 to 2 hours. Sweet potatoes take at least 2 times longer to bake than a regular russet potato. But when they are thoroughly baked, boy, are they yummy — makes a complete meal for me! ☺