After hearing the Sisters talk about this subject several times, I believe they would say that perhaps you are limiting their choices by having many of your classroom library tubs organized by levels.
Joan tells the story of a child in her class who wouldn’t choose books from the public library because there were no “blue tubs”. Yes, when we are conferring with our kids, we will give them good fit book according to levels. However, remember, those “3 ways to read a book”–by the pictures, retelling and reading the words.
So, you’ll provide them the books where they can read the words, and your library should offer those books that they’ve heard before (to retell) and books they can read by the pictures. It sounds like you have some of that going on, but perhaps take a look at “leveled tubs”, to encourage more choice or interest.
Here’s a link to a video of Joan talking about classroom libraries:
http://www.thedailycafe.com/articles/efficient-assessment-area
Here’s an article about classroom libraries: http://www.thedailycafe.com/articles/your-classroom-library

My library question is a bit different. Do you keep all your books out all year or do you rotate books in and out of the library? I keep the books I use to teach away and put them out after I am done so the students can read them but I wanted to know how others are doing it in Kindergarten. My room is small and I do not have an ideal place for all the books to have a premiere spot. I have read and watched all that I can on how to organize the books and will do that for next year when I am moved to a bigger (real kindergarten room) but still am unsure about having so many books out. Suggestions and thoughts?