It may be someone who is exhibiting signs of dissatisfaction or it simply may be time to promote an outstanding individual. While hard to accurately diagnose here in this forum, you will hopefully be able, with time, acutely focus your attention on certain individuals, at the right time. Whatever the case may be, once identified, you're going to take this individual aside and you're going to simply talk to him/her. Without passing judgment, you'll actively listen and take note. You may nothing in fact. You may have to come back to this person having given it some thought and come up with an action plan. More often than not, this plan will likely indicate that such a plan was unfortunately not already in place before problems started arising. In any case, this new plan will at a minimum indicate what the problem is, how you can go about fixing it, based on clear measurable results in a clearly identified time line.