Joe, sometimes on polyprop carpets you will have dust so badly embedded into the fibres it would not be removed by vacuuming, and if you have not loosened it your wand will not suck it out either. When wetted, this dust/fluff goes dark and really shows.
A duo will not touch it; either a rotary with stiff polyprop brush or a big CRB machine is the tool for the job. You'll see exactly what I mean, the first time you ever agitate a polyprop carpet with a proper agitation machine - it will be producing endless balls of black fluff that you didn't even know was there!!