You're having problems because the task is very difficult. Skirts are essentially impossible to conform because they don't follow the structure of the body -- two independent legs joined at the hips. In fact, all of the Poser skirts and dresses behave oddly when you move the legs, sometimes very oddly indeed. Because of this, some bonemakers have added extra joints so that the user can do things to fix the oddness, and others have given up on conformability and treated the skirt as a deformable tube. There is no approach that is anywhere close to perfect, and the best solution is a function of how much "freedom of movement" you need the model to have.