Why can't the Kindle display the front cover of the book I'm reading instead of the default artwork?

I'm reading a book that I bought on my device from the Kindle store. It was outside my normal genre choices, but I'm about a third of the way into the book and am enjoying it. The trouble is, I don't know what it is called. I know that I can go back to the home page on my Kindle and see, but it would be so much better if the Kindle were to display the front cover of the book I'm reading instead of default artwork when I put it down. Why does this not happen?