If your hubs aren't being read that often they will either have an empty circle or half filled circle. It has nothing to do with the quality of your hubs and everything to do with the views of your hub. Because I do not publish hubs for profit I don't tend to concern myself with my hub status.
Having said that if I were writing hubs to earn money the only concern I would have is if HP notified me that my hub was not of quality to be "featured" which means they aren't likely to promote it under (hub topics) or as a "suggestion" hub at the bottom of other writers hubs. Once again "no longer engaging" doesn't mean your hub is not "featured" nor does it mean it lacks quality.