Now that you’ve begun to use the Rug Doctor it’s probably too late for this, but it’s always a good idea to dry vacuum the carpet as well as you can before you start shampooing it. You can save money if you rented by the hour by vacuuming first, and also by pretreating bad stains with a spray type pretreatment and a brush. Turn your water heater to maximum temperature so you will have the hottest water available (turn it back when finished). Be sure your water softener has freshly regenerated if you use one. Then go get the Rug Doctor and shampoo away.
I’ve been making sure that any area I use it on has already been vacuumed multiple times.