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.