Often you have to key on their attempts to get into the confersation. If they start to mouth a comment, make it under their breath or start to raise a hand, they want in. Often we do not provide the space for the quiet students to "get a word in edgewise".

I often see my students mouthing the answers but will not speak up, so I look very carefully and say, "that sounded like the correct answer, can you say a little louder so everyone can here"  I have a small class so my eyes are wandering the room continuously!