There are some students who are so shy that they won't raise their hand to answer a question even if they know the correct answer. I never call on a student and "put them on the spot" if I think they don't know the right answer but you can often tell from a student's body language, the eye contact and maybe slight smile on their face, that they do know the answer. If I'm convinced that a shy student does know the correct answer to a question, I like to call on them so that they have that moment of "mini-success" and recognition. Sometimes their fellow students recognize the accomplishment with a nice comment or maybe even a round of applause. This might be a little bit embarrassing for the shy student but I think it makes feel like they are now really part of the group.