Take you time. Keep track of the hourly rate. This is used to ensure many billable contact hours. The purpose is to ensure that the child who is low functioning, or who you want to be, does not understand the task. To provide additional instruction to the child who has failed one of the items would only confound the results.
Teaching simply serves to frustrate the child and to take more time.
Think of the questioning as a clinically important sign. Don't feed into this. If you believe that the child knows the answer to the question, fail them on the item anyway. Do not give credit if the child responds correctly after they have given an incorrect answer. When needed repeat this command often and increase the threat from each.