This can happen if someone holds the slide in place. Imagine firing a semi auto pistol, each time you fire the slide blows back and then returns forward. This action advances the next round into the chamber to be fired.
Ayoob makes the case that during a violent struggle over a handgun, Martin's hand may have been on the slide preventing the next round from loading when Zimmerman fired.
That sounds reasonable. What else? Magazine... Ok, I like calling the thingy that holds the ammo a clip. I grew up that way. I know better than the old rifle, gun thing, but clip just seems right.
The bullet holding thingy wasn’t set, or the gun just jammed.
Either way, wouldn’t the slide lock back? Zimmerman reholstered and waited for police. The police cuffed him and took the weapon from the holster. Zimmerman would have had to release the slide back, right?