NodeMouseClick event not working correctly?

I am using treeview to display files and folders like Windows Explorer. it has a NodeMouseClick event, but sometimes when I click +, that event doesn't fire.

private void treeView1_NodeMouseClick(object sender, TreeNodeMouseClickEventArgs e)
    {            
        MessageBox.Show("node mouse click");            
    }

      

        
        
        
      

    

Can anyone explain to me why? and how do I know when I press +?

early!