Next button becomes disabled when returning to slide

I have an issue where the next button returns to disabled when I use previous button to go back to it. I have the slide so that it resumes from where it left off. 

The way I have it set up is, the 7 buttons on the page must be visited in order to make the next button normal. This works, but if i return to the slide the next button returns to disabled and I am unable to get it to return to normal.

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