Previous and Next Button missing on 1 slide.

Here's how you can add triggers for the next and previous slides:

1. Click the Create a new trigger icon in the Triggers panel, or go to the Insert tab on the ribbon and click Trigger. 

2. Set Action to "Jump to slide" --> "Next slide" (change this to previous slide for the previous page trigger).

3. Set When to "user clicks or swipes" --> "Next" (change this to previous for the previous page trigger). Click OK.

Repeat these steps for your previous page trigger. 

Here's how it should look like for the next slide trigger:

For further reference, you may check the following page:

https://community.articulate.com/series/articulate-storyline-360/articles/articulate-storyline-360-user-guide-how-to-work-with-triggers

Hope this helps!