two things
soiled poly begs for mechanical agitation for best/fastest/easiest results
strong firewater chems, long dwell and mega heat (240+ ) can make up for lack of mechanical agitation too on ploy
That's the "soil" part of it
the other thing, poly cut pile is plain crap and "dulls out" different than nylon cut pile
the fibers abrade and get dull looking unlike nylon that shows wear, but doesn't "dull/dinge" out like ploy crap
ain't squat I know of we can do about that
..L.T.A.