Outline navigation not clickable

I am using Storyline 2 on a Mac running Parallels. When I publish a story, some of the outline chapter titles are not clickable. The previous and next buttons work fine. 

My story file has mp4 files on slide and each scene has one slide.

This is happening for both the flash and html file versions. I can't figure out why some titles are clickable to go to the slide and others are not. I'm stumped.

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