Its not confined to just P5 figures and clothing. The only requirement is that the clothing must not touch any of the figure when at the zero pose. Some P4 garments have capped sleeves and legs. P5 works on collision detection, since these garments are already colliding with the figure they will not work. The solution would be to edit out the capped ends either with the Poser grouping tool or in a 3rd party 3D program.