The interviewer wants to see that you’re a critical thinker and an effective problem solver. Even if you don’t have a specific example to talk about, explain that you always care about your team members and would want to uncover the root of the problem, and find out why, exactly, they’re unhappy. While it may not be possible to please every disgruntled employee, the interviewer just wants to see that you would make an effort to rectify the situation and be professional about everything.