I haven't had this problem yet, but I agree that accountability is the problem. I like Michael's suggestion because it prevents individuals from hiding behind their team. You may want also to consider grading some team activities. My students don't know which activities (individual or team) will be graded and which will not be. Another suggestion is to leverage peer pressure to get the other teams to goad the slackers into action (i.e., If we have to do it, so do they.) I would do this with a little public shaming (although you have to be careful to do this delicately so it doesn't backfire on you). I've done that before with a team that performed horribly on a RAT due to lack of preparation and they were so embarrassed by the reactions from the other teams that they aced the next one.