I would stop using that center while you start over teaching the
procedures and behaviors for the library center. Teach them the 3
ways to read a book, what the behavior looks and sounds like. Have
some children model the correct and incorrect behavior in the
center. Have everyone practice the correct behavior for several
minutes and bring the group together to talk about the experience.
If you have read the Daily 5 they need build "stamina" over time -
starting with reading to self for only 3 min. and building it up
over time. They may be there too long to stay engaged in reading.