I want you to think about that statement in relation to where you are training your dog. Consider the environment you are training in and what additional learning your dog may be experiencing as you train together. Can your dog learn easily, and have you set them up for success? Are there other dogs too near for him to successfully perform for you or do you need to be more considerate where you are training your dog? Maybe what you are teaching your dog demands individually training him in that environment.
Look around, consider your training location, and remember dogs are always learning!