Thanks for playing! We've had survivors turn very reliably in testing, so you were probably just very (un)lucky. Every time a survivor is downed they get a stack of infection. During combat, every 15 seconds each stack executes a 10% chance to transform the survivor - so the chance compounds fast.
I've left the weapon on semi-auto, because it was way more satisfying to pair the mouse clicks with meaty sounds of impact. I also feel it adds tension to the combat. The game is short enough for this to not be very straining. Had we managed to squeeze some sort of weapon handling/reloading mechanic in, I'd likely go for automatic fire.