Sounds like you are trying to take too big of flake, striking into the piece too much , striking to far back. Sounds like you are getting deep heavily bulbed flakes, that are hinging out.Try to hold the spall for now with its center plane nearly level, Strike your platform strait down, on the edge. Remember that the face of the platform angle needs to be less than 90 degrees. grind platforms pretty well. Pictures would be great.
If you are spalling chunks the same applies.