When you are running a free discussion class for ESL English Learners, and although the topic is pretty controversial, still some students do not talk or express their idea within more than one or two sentences. What are the solutions in this situation? What are the tactics to keep them going?

For example, the topic is about marriage (the best age to get married, customs and everything related to marriage), then all the things some of the students would say are:

Tom: Well, I think the best age is between 25-30. And I think the boy and the girl should get engaged for at least few months.

When you ask the student; why do you think this is the best age? They simply answer: because they are fully grown up and probably finished their education.

And the more questions you ask, the shorter answers the students give. And the flow of discussion do not go ahead as you want to be, and it turns mostly into a question-and-answer talk show.

For some students, thanks God you don't need to burn lots of energy and they seem to be a real chatterbox and they will talk about so many things and even to the extent of going astray which you need to stop them in a way that they don't mind it, and bit by bit remind them to get back on track and continue their talking about the discussed subject.

So, I need some help and good references to show me how to increase the motivation of such students who do not talk a lot.

Thank you.