New Potatoes with Roasted Garlic Vinaigrette
Credit: Becky Luigart-Stayner; Styling: Leigh Ann Ross

In all likelihood, you are not giving them the space they need. Either you are "boxing them in" by trying to roast them in a pan with high sides, or you are steaming them by placing them too close together on the baking sheet.
When food cooks, it gives off liquid in the form of steam. You want to give that steam the best and fastest possible escape route so your potatoes crisp rather than steam. The secret to success couldn't be simpler: place the potatoes in a single, un-crowded layer (they shouldn't touch each other) in a pan with very low sides. A jelly roll pan or sheet pan, is ideal.

And remember, the potatoes will brown on the underside first, so turn them when the underside, not the top, is golden.