How do you handle small group mutes?

It's every small group/bible study leaders nightmare.

No. One. Talks.

Under numerous circumstances, your group can become a pack of mutes - 
A group of year 7 boys who haven't developed the ability/vocabulary to share more than a grunt or fart jokes...
A group of year 9 girls who are in the middle of the latest "crisis" in their "social group"...
A newly formed bible study group...
A bunch of introverts...
Perhaps your usually talkative group have all had a hard week at work/a stressful week at uni/a tough week at school...

When this happens, what do you do? 

To some extent, this quietness may just be a characteristic of some groups and is nothing to be really concerned about.

For some members of a group, they will be fully engaged/thinking internally, yet outwardly quiet. This might have to be something which you'll just have to deal with.

In some cases, it may be a result of group dynamics, where primarily the extroverts talk, or at least get their responses/opinions in first.

No matter what the cause, what do you do if you're the poor soul trying to direct the group of mono-syllabic members?

First, you can just make sure they write down their answer, aware if it will be shared or remain private.

Second, you could give the group notice that you're going to give them all 30 seconds to think about their answer before you want them to reply. This may give the introverts a chance to get a word in.

Third, you could have the members pair off and either share their collective answers or the best part of the response from the other person.

Fourth, you could ask what they would add to the answer they heard in a small group.

Fifth, with or without advanced notice, you could call on people by name to give input. This is dangerous if the group is always quiet, but effective if it's an unusually quiet week for a group of regular chatterboxes.

Sixth, you could ask the members to give responses that they think those they know might give, like their friend or family. This gives the additional advantage that they can share their own answer, but under the guise that it's from "someone else."