Help! My hidden next button is not visible when user returns from last scene

Four or five seems to be the limit of the number of objects that will work with "When state of..." trigger. What you most likely have to do is to use variables to set Next to disabled and Normal.
Create a variable for each page/scene to be visited, and at the end of that page/scene, change the variable to true. Return the menu slide to initial state, so the timeline starts. Then set Next to Normal when the timeline starts, if all page/scene variables are true.
Resetting the menu to initial state will cause a loss of "Visited" state, so you will need to create your own state. I call mine "Completed". When the timeline starts, the icons which have been visited (variables are true), are set to completed, and if all are true, Next is set to Normal.
There is a sample of how this works at this discussion: https://community.articulate.com/discussions/articulate-storyline/free-sample-restricted-and-free-random-navigation-using-variables-and-triggers There are three different levels that allow the learners to choose when to visit which topic, but they all show the custom Next button when a predetermined number of pages have been visited.