non-vB HTML question

[dismas]

I'm trying to trouble shoot something for someone and I can't see where the problem is. Could someone look at this page and tell me why the bottom "Wildlife" button isn't showing up until you roll over it with your mouse?

Here's the CSS for that page:

#naturallightgallerybutton a:hover  {
   background-position: -622px 0%;
}

#studiolightgallerybutton a:hover  {
   background-position: -595px 0%;
}

#wildlifegallerybutton a:hover  {
   background-position: -572px 0%;
}

#studiolightgallerybutton a  {
   background: url(sg_gallery_media/studiolightgallerybutton.jpg) no-repeat 0% 0px;
   display: block;
   height: 369px;
   width: 595px;
   font-size: 369px;
   line-height: 369px;
}

#naturallightgallerybutton  {
   visibility: visible;
   position: absolute;
   left: 63px;
   top: 674px;
   z-index: 3;
   width: 622px;
   height: 358px;
}

#studiolightgallerybutton  {
   visibility: visible;
   position: absolute;
   left: 70px;
   top: 271px;
   z-index: 3;
   width: 595px;
   height: 369px;
}

#wildlifegallerybutton  {
   visibility: visible;
   position: absolute;
   left: 69px;
   top: 1066px;
   z-index: 3;
   width: 572px;
   height: 358px;
}

thanks!