I have a TreeView and I have assigned an ImageList to the the TreeView:

mlstImg.Images.Add(Resources.EMPTY);
mlstImg.Images.Add(Resources.MIXED);
mlstImg.Images.Add(Resources.DOWN);
mlstImg.Images.Add(Resources.UP);
this.TreeViewSites.ImageList = mlstImg;
this.TreeViewSites.ImageKey = null;
this.TreeViewSites.SelectedImageIndex = -1;
this.TreeViewSites.SelectedImageKey = null;

The problem I am seeing is that sometimes an image is not displayed for a node, yet I know each and every node is assigned an ImageIndex.

Why would sometimes an image not be displayed?