Looks like the board isn't registering the stab motion. I've found a short, sharp motion away from the blade end has the desired effect most of the time... short video:
Anyway, the board seems to register the "draw back and stop" part of the stab motion, rather than the "thrust" part, so you might try holding the saber vertical in one hand, and pressing and holding the switch with that hand. Then gently tapping the pommel upward with the other hand. This triggers the font selection menu both Diamond sabers I have available.