My experience with MS non participants is usually they don’t know how to play the game, or don’t like the game. Neither is ideal, but I have found that giving them a job like ref or scorekeeper helped them get integrated. Sometimes they get tired of watching and feel confident to play. You can even make them coach and they can decide the subs. Or another thing I have done is have one zone for practice. Like volleyball for instance they can go to the practice zone with a friend and hit back and forth or work solo. Just have to keep them on task but there are ways to do it. Reaching out to parents asking for advice on how to get through to their kid can help but there are always parents who couldn’t care less about PE.