Attach the flying line 1/2"-ish up more towards the nose of the kite, so that the wind slides past it more easily, rather than pushing directly on the sail. When the wind deforms the shape of the sail too much the kite won't fly.
@ACrop You will not be disappointed. Even pilots who have been flying for 10 years and more can learn something at one of JB's clinics. There is always something new and different happening with kites and John is always on the cutting edge.