No NEXT Button When Click PREVIOUS

It sounds like you have a trigger that disables the Next button when the timeline starts. That trigger runs when returning to the slide. 

Since you don't want it to run if the user returns to the slide after viewing all the layers, you need to add a condition to prevent the trigger from disabling Next again. 

Here's an easy way to do that: 

• Create a T/F variable with a default value of False. 
• On the last layer, add a trigger that changes the value of the variable to True. 
• For the trigger that changes Next to Disabled, add a condition so it only runs when the variable =False.

If that's not the problem, well, further troubleshooting is just guessing without seeing the .story file. If you upload it, someone might be able to solve the issue.
Here are the best practices for uploading a .story file:

• Only include slides that are related to the problem. In this case, that would be the one slide with the problem.
• Be sure objects, layers, motion paths, and variables have meaningful names.
• If there is proprietary content, replace or delete it. For example, replace proprietary text with “ipsum lorem” text.