Do you mean little bits of fabric wich are stuck on the carpet? Also known ase "sock dye" you need to look close for this cos from a standing postion it might look like a greying traffic lane.
This area in the lounge is the most trampled on area, so there will be alot of wear and tear. Which cant be fixed, just make sure u point it out to the punter before u start.
You can try adding a booster to your pre-spray like citra or enegiser etc that should help with the greying/traffic lane/sock dye (a bit) IF it has those "bits" of sock fibers in the carpet like i mentioned before, then a stiff hand brush used in a light circular manor should make things better... think Karate Kid wax on wax off lol
Tony