Make sure the trigger area pixel (which is marked red in the graphic) is inside the ground, not one pixel above it. Otherwise, they immediately change to fallers (and then walkers) due to having nothing to stand on. Earlier versions of NeoLemmix auto-adjusted this to some extent. Based on discussion with Nepster and Simon, in particular the point that "the software should not try to be smarter than the user", this adjustment no longer happens with newer versions.