I was nesting parallelism with cilkplus. At the high level I invoked several cilk_spawn, inside of that I then used cilk_for. I created a reducer - there doesn't seem to be any way to reduce across the spawned tasks without going back up to the function invoking the spawns and calling cilk_sync. This means I must invoke cilk_spawn with new entry points to continue.
Something like this:
cilk::reducer< cilk::op_add <int > > mysum (0) ;
. . . .