Eventually they learn to anticipate the type of questions needed to produce better art, and they will need less hand-holding. Your teaching can empower them. Other students are handicapped by being very slow and deliberate. They may be perfectionists because they are afraid to make a mistake. They need confidence to experiment with expressive approaches.
Sour lemons make great lemonade with the right additions.
Empower them by building their confidence. Don't encourage these students to start over unless they have a better idea they are anxious to try. Rather, encourage them by pointing out that these things are best learned by practice and the more we practice the better it will get. Find the best part of what they have done and tell them what parts of their work are best and explain why you think so. Don't use praise that is empty or general, but praise together with specific information so they can learn from it.