There are a number of reasons why students are reticent to talk. I think there are 3 major causes, so those are the ones that I’ll outline here.
First of all, having the necessary language is important. Think about vocabulary, functions and grammatical structures that are likely to occur in the speaking activity. Present and / or revise these points well in advance and review pronunciation. You might also want to keep some of the points visible on the whiteboard as a reference.
Information about the topic is another factor. Students need to feel that they have something to say before they start speaking. It’s important to do preparatory work before expecting students to speak. You might want to give them time to think about and plan what they are going to say. You could also do a listening or reading activity that is related to the topic to give students ideas. This is especially useful if the activity is of the debate type.
Finally, the atmosphere is important. Students talk more easily if they are in a non-threatening situation. It sounds as if you are concerned about your students, so I imagine there is a good atmosphere in your classes. Make sure that students are respectful of others and not laughing at others’ efforts. You also need to get the balance right regarding correction. You need enough correction for the students to be learning something and not too much so they are not inhibited. Small groupwork and pairwork also helps students who do not like talking in front of the class.
You could also look at my response on to “Webmonkey” on December 5th which might help.
Thanks for the comments. I think it’s great that you take these opportunities to find out more about teaching.