Selected Index Not Changing

I have a couple pages that are all pretty similar. Each of these pages consists of some number of views. Roughly speaking the first four views on each page are the same. The first page has a listbox and a textbox, then next several pages have listboxes. Because these views are the same between all the pages, and because the code behind them is the same, I built up the first page and got it working, then copied and pasted the information over for the other pages. The pages all differ in what the textbox does, but the listboxes are all identical in every way.

So, what I am seeing is that the original page works perfectly. When I go to any of the other pages, the behavior is bizarre. I was a bit surprised to find when the SelectedIndexChanged event was raised for the listbox, but it's fine. In the SelectedIndexChanged event, some minor stuff needs to happen as long as the selectedIndex is not -1. I am getting the SelectedIndexChanged event when it should happen. I see this by putting a breakpoint in the event handler. The SelectedIndex is ALWAYS -1. I can't get the SelectedIndex to be anything else, though it works in the original page from which the code was copied.

Naturally, I assumed that I had overlooked some change in the copying and pasting. That doesn't appear to be the case, but it still makes the most sense. The event is triggered by a change in the selectedIndex on the page that is currently loaded. However, the SelectedIndex hasn't changed because it was -1 and is still -1. The SelectedItem is still nothing, as it was before. There also isn't any problem that I can find with the copying and pasting. I even tried renaming two of the listboxes on one of the pages so that they would be truly distinct from the page that was working (different ID, which shouldn't matter since they are different fields). This had no impact. Nothing I can do will cause the SelectedIndex to be anything other than -1.