What do you do when you get the blue arrow that tells you traffic on a hub is declining?

I don't really do anything, but that's because most of my down arrows appear on my poetry pages. It just means traffic is doing what is natural, it rises and falls. Sometimes there will be searching a lot for a particular topic, sometimes they aren't. If you want to do something try changing the title slightly, or rewriting parts of it, or making sure it has the most accurate tags. All of these will force google to review it and you could get more page views that way.