If it is always the same people who refrain from speaking whatever the method or topic, it is important to consider why they may do so and to compare this to their behaviour during informal periods. Whatever it may be, it is important not to try to guess the causes but instead try to encourage participation. If the issue persists, set aside some time to discuss it in plenary. People are often unaware that others may see their silence as judgemental. A plenary discussion may bring this to light and help remedy the situation.