Are you par-boiling them before roasting? If not that is where the problem lies... Heat oven to 180˚C and have a roasting pan with fat, not oil, ready to go in. At the same time par boil your potatoes for approx 5 minutes then drain. Once oven is at temperature and the pan with the duck fat or similar and heat. Tip the potatoes into the pan and shake around to coat with the fat and fluff up, cook for around 45mins., possibly a little more.

My partner will not have any other spuds so King Eds are all I ever use. As Moongrower says, boil them for about 5 mins before roasting. I always use olive oil and have many compliments on my roast potatoes which are fluffy inside and crunchy outside. Put a little salt in the par-boiling water and shake them in the dry pan or use a fork to create an uneven surface. Then hot oil Good results guaranteed.