360 Review - freely navigate

It sounds like the course is set up with Restricted navigation. That setting automatically disables the Next button until a slide's timeline ends (but only the first time a user views the slide). It also prevents the user from advancing to a slide by using the built-in Menu. So unless the course has custom navigation buttons, the user will have to go through the slides in order.

That navigation setting is in the Player. If you change it to Free navigation, reviewers will be able to use the Menu to access any slide.

FYI, even for a course that will use Restricted navigation for the end users, I usually switch to Free navigation for the Review. And if the course won't use the built-in Menu, I turn that on. I also display a prominent note at the beginning of the course to let Reviewers know how navigation was changed for their convenience. 

More FYI: Here's a post about different ways to control the NEXT button: https://community.articulate.com/discussions/articulate-storyline/tip-controlling-the-next-button-101