The skirt will not conform like the rest of the clothing. That's just the way it is with skirts in Poser.

Instead, a well-made skirt has "pose morphs" in it. It lets you morph the skirt to fit the pose. Look in the skirt's body, hip, or skirt part. There should be dials for pose morphs somewhere, at least for well-made items like Pretty3d and Wusamah's stuff. Dials with names like LeftLegSide, RightLegSide, LeftLegBack, that sort of thing. Maybe Sit or Kneel dials, too.