You've discovered why dresses shouldn't have thigh and shin groups. :-) They end up behaving like pants. It's a limitation of Poser.

You cannot make long skirts that conform to the legs. I think you have to re-group the skirt so only the hip conforms. Then add some pose morphs to the skirt (walk, sit, kneel, etc.). The user will have to use the dials in the skirt to match the pose.

Either that, or make it dynamic.