If a student starts to struggle throw them a life line by giving them an option of phone a friend. Student simply picks another student in the room to answer the question. You can allow the student to ask other students if they know the answer or let them cold call a friend for the answer.
If you use this strategy but never hold the original student accountable for the answer students will learn that if they don't know the answer in this class your left off the hook for even trying (Classroom Culture). Therefore, to make this strategy to work it is highly recommended you incorporate the no opt out strategy along with phone a friend.