They can train around the weak spots. Even the best-designed training materials will have some weak areas. Good trainers can spot these and develop ways to work around them. Top trainers know that you can't teach a sleeping student, so they work diligently to present their information in a manner that keeps trainees attuned, alert, and learning. They are eminently adaptable.
Flexibility is the key to good training.
What works for one student may not work for another and trainers know how to adapt their styles, examples, and material to fit the needs of different audiences. They are time bandits. One key to successful training courses is to start and stop on time. They are masters of their domain. Once in the training environment, the trainer controls everything.