This slider's previous and next functions fail after the user progresses through the initial section and then selects a new section. Any idea why?

So you don't miss them, the prev/next buttons are the edges of the prev/next slides which hang on the left and right edges of the page, respectively. And to add further clarification, when you get to the 4th slide, you can then select your next section. After you do so, the prev/next functions fail.

JSFiddle

Thanks