Tell me about a time when you gave someone an assignment and he or she didn't live up to your expectations.
How did you know there was a problem?
What did you feel was the cause of the problem?
What guidance did you provide this person with, once you knew there was a problem? When did you provide this guidance?
How did you motivate him or her to do better in the future?
Given this individual's performance, what type of assignment did you give him or her in the future?