Often that means it has gone idle (the black "H" disappears). That means that is hasn't had enough views and so it hits an idle status. There are a couple of things you can do. You can edit the hub, or if the hub is just the way you want it, all you need to do is to make one or two little changes....add a comma, or add a copyright notice...any small change and then republish.
A lot of my hubs are poetry and the text is exactly as I want it, so I do not want to edit those pieces. So I make a tiny change to my copyright notice, add a word or change punctuation. Then I republish, and in about 24 hours the hubs are up and featured again.