I dedicated an entire year of knapping to learn how to avoid step fractures. It's not easy.
The biggest challenge for me was my lack of good stone. I recommend trying your technique on high quality flint like Hornstone or Georgetown to see what's going on. If you can't avoid step fracture on that material, you've got to put more aggression into it.
Step fractures are caused by poor quality stone, lack of force, force applied at the wrong angle, or lack of solid contact with the platform as the flake detaches.