Have you tested your water? Are you sure the sword being chased is not a female? Platies and swords can interbreed. Occasionally, platies can be punks -- I had a colony of X. variatus once I couldn't put with anything. If the sword is joining in tho, I would explore other reasons. What are the dimensions of this tank? As has been suggested, have you tried breaking up the fishes' sight lines a bit?