I am not a drum corp instructor or anything or a band director but these are a few things that visual techs have done during basics.
Break down each step, for example just do one step and have them stay there, this enforces the idea and do that a couple of times. Then move to doing two or three steps. I know that has helped me out to becoming a better marcher. I second the point that some people just can't get it, but another idea is doing some one on one work with people who are having trouble with it and maybe having a veteran or their section leader come and help and give advice. It might help to hear it from someone who went through the same thing. Maybe also get a special volunteer like someone who has marched drum corps or like a director of a very good college band. I remember in band camp our band director brought in the band director from West Chester University, and he helped us out alot, and gave us really great tips some of us never heard before.
Hope this helps!