I also find it helps to consciously think of pushing the shoulder blades down into the step while you are doing the pulling motion. That helps me focus the effort mostly on the back (though the anterior shoulder, triceps and chest will also be involved somewhat). If you are still having difficulty focusing on the back, try doing a set of 1-arm rows or barbell rows immediately before doing the pullovers, to pre-exhaust the back muscles. Often, it's easier to feel them working, then, and easier to put the mind into the right muscle.