At some point, the students must be held accountable for the participation (or lack thereof). The teacher should continue to ensure that students understand their lack of participation is negatively affecting their grades. The teacher might conference with each of the students individually, and demonstrate how the negative scores impact their grades as well as developing a plan with the students to encourage participation in classroom discussions.

The teacher should document each time the students are not participating in the discussions. If after meeting with each student individually and showing them the impact on their grade doesn't help, then contacting their parents and meeting with them may.

The teacher should encourage the students to answer the questions, they should also show the student some possible solutions or answers so they know exactly what the teacher expects.

Differentiation is something that our school is focusing on tremendously right now. You could find a way to grade them by their contributions even if they don't express them themselves. For example, you may have students work individually first by having them write down their ideas about the discussion topic along with their evidence supporting those ideas. Then, partner each student with someone else,(preferably someone that is more vocal than they are. When you enter into the discussion, one or both students may contribute verbally, but if only one does, they will be sharing their ideas as well as their partners.