Javascript for navigation buttons at top of page not working on some pages

I used Javascript from this post to create Next and Back buttons at the top of my surveys. It worked without any apparent problems in a test survey I set up. However, when I applied it to the final versions of two different surveys, the buttons do not work on certain pages; clicking it turns the button white, and not action is performed. I can't seem to isolate the problem; there doesn't seem to be any type of item or customization that exists in the pages or blocks where the buttons don't work that doesn't also exist on pages where the buttons do work.
Is anyone aware of things that might interfere with the functionality of this Javascript, like other Javascript in a header or question, other content in the Admin theme, Display/Skip Logic, Look & Feel headers, custom validation, etc.?