This is usually the result of custom node icons having been installed while neglecting to set appropriate background positions for the image. The default node icons come from a sprite image with background positions. If you, for example, replace the sprite image with a non-sprite image then it is necessary to set background-position: 0px 0px; in your CSS. Otherwise you get a situation where only one status works correctly (read vs unread icons).
So basically... style problem. If you aren't sure where your custom CSS is then post the forum URL and I will debug it.