Is someone failing to meet all of your expectations? If so, the problem could be with your expectations. You can either choose to rethink what you want out of this person, or make it clear to her or him that the current situation just isn't going to work for you for very much longer. Your disapproval might be just what this person needs to make some real changes for the better. Use your power to motivate, but make sure you aren't holding anyone to unrealistic standards.