Unhide Next button when revisiting slide

The trigger that hides the Next button needs a condition so it will only run the first time the user visits the slide. 

Based on your description of the slide, you could do this: 

• Create a T/F variable with the default value of False.
• For the trigger that hides the Next button, add a condition so it will run if the T/F variable = False. 
• Add a trigger to the slide that changes the T/F variable to True when the media (that is, the video) completes. 

Here's more info and a demo for controlling the Next button in different situations: https://community.articulate.com/discussions/articulate-storyline/tip-controlling-the-next-button-101 

It’s worth the effort to learn about variables and trigger conditions, because they provide the real power in Storyline. Here’s more info:

• https://community.articulate.com/discussions/articulate-storyline/the-value-of-variables
• https://community.articulate.com/series/articulate-storyline-360/articles/articulate-storyline-360-user-guide-how-to-work-with-triggers#conditions