Organizing favorite websites?

When I started bookmarking favorite websites, I was able to organize them by category--language, religion, health, etc.--and add new favorites to existing categories. Just recently, however, when I click the "Add to favorites" button, nothing seems to happen, but it turns out that those sites are being added to the "Favorites Bar" at the end of the list. How can I move them to the categories I want to list them under?