Additionally if the Navigation Restrictions were set to "Locked" within Menu Options, as soon as you navigate to the next slide, the previous slide gets locked. I suggest you to change the setting to "Restricted".
With this option enabled, initially the Next button will be disabled and upon completion of the timeline, the Next button appears allowing the user to navigate to the next slide. Once the slide has been viewed, you can observe that a tick mark appears next to Menu title within the Navigation pane, which allows user to review those slides that were previously completed.
