looks to me like its worn out already. maybe something they track from outside. might be damaged beyond repair. I clean some offices where whatever they track from warehouse just doesnt clean up anymore.
You can try an anti-browning agent like citric acid along with a speed-dry to see if it's simply watermarking. If not, a petroleum-based booster added to your prespray may help IF the soil is actually coming from the warehouse.
thought so, I did one where they re threaded semi's tires and some black stuff just wouldnt come out. Some carpets just wont be restored to brand new appearance. I make sure to tell people before I start the job if I think there will be issues like that. They usually understand