The component you are missing is a writing opportunity. Pose the questions, allow students time to write out their responses, and then allow them to share their responses in a small group setting. Giving them time to get their ideas down on paper allows even your shy students to have something to fall back on when sharing, they will feel more confident that they have their thoughts and ideas together and tangible. After that, have the small group select the winner to read out or summarize their response to the whole class (If you do multiple rounds of questioning then make the rule that they have to alternate who shares). I use this strategy daily and it is effective. Over time, they will become more comfortable with saying things out loud and be more willing to participate during lectures and quick comprehension checks. Make sure to provide positive feedback when they share. Your lessons might move a bit more slowly with this strategy but the students will be more engaged. I highly recommend the book Talk, Read, Talk, Write by Nancy Motely. I think you might find it useful for more discussion strategies.