XMenu Icons Not "Unique"

The icon for the user-defined XMenu that appears in the menubar is a generic folder, whereas icons for all of the other XMenus in the menu bar are unique.

Similarly, although all of the items (all of which are aliases) in the user-defined XMenu folder are unique, only some of their icons show up that way in the drop-down user-defined XMenu in the menu bar—the others appear as generic folder icons. The same is true for the items in each of the other XMenus (for example, some of the icons in the drop-down “Applications” XMenu are unique, while others are generic).

(I have the “Unique” box checked in the XMenu prefs.)

What should I do to have all of these icons show up as “unique?”