Put the blade tip towards the sky and stab upwards. I don't know why but that always works best for me. Hold the button and stab the sky.
You can also turn up the sensitivity for the menu interface in the ultrasabers editing program. The larger the number the more sensitive it will be.
I'm getting the stab sound effect pretty consistently (moreso with the upward motion, so thanks for that tip), but it's still going to flash on clash or powering down when I release the button. I'm also not getting any indication in the tool when it's connected to the computer (even though other things work on the same USB port for me and I've tried multiple micro USB cables in it, so I haven't been able to adjust through there yet either.