You'd think if you tell the people who are reporting to you to do things they'd do it, but actually, you have to convince people. We all have influence, not just the people on the top. Influence can be positive or negative. Even if you think no one listens to you, they probably are. However, positive reinforcement is useful for getting people to do things.
Negative reinforcement is only good for getting people to stop.
If you are too negative, people won't do anything at all. They react like mice in a psychology experiment who, when subjected to too much negative reinforcement, ended up huddling in corners. This is not the best use of your volunteers. On the other hand, it is necessary to be truthful. Whatever else may be going on, the person you are talking to tried their level best to the best job they knew how to do.