You could try tailoring discussions to those student's hobbies and likes. Students who are reticent most of the time actually tend to me MORE outgoing when discussing a topic they are very interested in. Doing this once or twice may "break the ice" with that student and get them more interested in discussions in general.
Also, some students are just not comfortable in large groups. Perhaps you can try and get them involved through writing reflections or other kinds of solo activities instead.
