From the limited amount of information you have provided, it's impossible to tell exactly what caused the death of your Knock Out roses. They do prefer full sun, so that shouldn't be the problem. If your soil is heavy and poorly drained, that might be the problem. Try growing the roses in raised beds amended with compost to loosen the soil. Roses also like to be kept evenly moist. Make certain that they get adequate soil moisture, especially for the first growing season after planting, and through the heat of summer.