Dimming problem in slide show

[eobiii]

I created a slide presentation to present for an annual meeting of a plant society, but when I preview the slide show, some, only some of the slide dim out in a couple of seconds after appearing. They don't become completely dark, but are noticeably darker than when they first appear and darker than the original file, or as they appear in the slide sorter view or the normal view. And it is only happening with SOME of them, not all. I tried removing all transition times and am using fade with a mouse advance. Any ideas what is causing this?