blank slides in sliding panels

I used to be able to insert blank slides into the sliding panels theme. I did this all the time to manipulate the slideshow to display the way I wanted, and to create chapter breaks. I have a few published slideshows that prove it. Now, however, the function is greyed out unless I'm in Classic or Ken Burns themes. When I open my working files of those published slideshows all the places where the blank slides were (originally black to match the background) are white boxes. Anyone have any idea what happened?

-Matt