Remember, however, that students have the right to do mediocre work and to be graded accordingly. You need not take it personally or let it interfere with your work. This should be a matter of class policy, discussed in advance with the instructor. If several teaching assistants participate in one class, discussions before the beginning of class and during the quarter can help define expectations of the supervising instructor and address techniques which may help avoid lack of participation. For example, distributing sample questions or topics for discussion at the next session may help promote discussion.
Several options exist if a discussion is slow to develop.
First, be prepared to give a mini-lecture, although this option may encourage students to be passive in the future. This option depends on your advance preparation. A second option, if students persist in their negligence, is to consider early dismissal of the entire group, or only those who are habitually unprepared. The risk with this action is that students may become alienated rather than motivated to improve. The best option is to refuse to allow lack of participation.