Unlocking a custom 'Next' button after user visits each layer

I'm not on my PC, so I can't look at your file right now. However, I'll offer these suggestions/tips.

• If your POI buttons each have a Visited state, a trigger can change the state of the custom NEXT button to Normal when the state of all the POI buttons = Visited. 
• You don't need a trigger to hide the custom NEXT button when the timeline starts. Just set its Initial State to Hidden.

• This post includes a file that demonstrates various way to control the NEXT button. It's about the Player's NEXT button, but the same instructions would work for a custom button. https://community.articulate.com/discussions/articulate-storyline/tip-controlling-the-next-button-101