问为什么FileSystemWatcher不触发事件？

public void Setup() {
    var fileSystemWatcher = new FileSystemWatcher(_myFolderPath);
    fileSystemWatcher.NotifyFilter = NotifyFilters.LastAccess | 
      NotifyFilters.LastWrite | NotifyFilters.FileName | NotifyFilters.DirectoryName;

    fileSystemWatcher.Changed += FileSystemWatcherChanged;
    fileSystemWatcher.Created += FileSystemWatcherChanged;
    fileSystemWatcher.Deleted += FileSystemWatcherChanged;
    fileSystemWatcher.Renamed += FileSystemWatcherChanged;

    fileSystemWatcher.Filter = "*.*";
    fileSystemWatcher.EnableRaisingEvents = true;
}

private void FileSystemWatcherChanged(object sender, FileSystemEventArgs e)
{
    MessageBox.Show("Queue changed");
    listBoxQueuedForms.Items.Clear();
    foreach (var fileInfo in Directory.GetFiles(_myFolderPath, "*.*", SearchOption.TopDirectoryOnly))
    {
        listBoxQueuedForms.Items.Add(fileInfo));
    }
}