Button won't work

I can't get this back button to work, no matter what I try.

There's a button that runs an advanced action that shows another button and a square. The button that appears has an action that shows a checkmark and a continue button. All that works.

However, I have a back button showing at the start of the slide, which doesn't work. Pushing it doesn't do anything. Pushing it several times continues to the next slide. I've tried "return to previous," "jump to <the previous slide's number>," and making an advanced action that goes to the previous slide. I've also tried deleting and recreating the back button. It seems like such a simple thing, but it will not work no matter what I do. 

Mark -