Sometimes a member of a group will be silent for a while, because he's disengaged. That might happen because the question is too hard or too easy, because he's feeling like his contributions aren't being valued, because he's being distracted by something else, or for other reasons.

Sometimes a simple question directed at a silent member can get him back into the conversation. Or sometimes it will just let you know what's going on.

A: C, do you see any problems with the approach we're taking?
C: Sorry, my sister spilled Coke all over my desk
C: and I've been cleaning it up...