I've been messing with my Serpa and I've found that I can make it jam every single time. To make the jam happen just pull up on the gun, like you're drawing it, before you press the button. With upward pressure on the gun the button will not depress and it won't release until you put downward pressure on the gun and press the button.
Of course this is what it's supposed to do, prevent somebody from pulling the gun without pushing the button. If I lay my finger on the release button as I get my grip with downward pressure on the gun it releases like butter...every time. The training vids I've encountered all say to come down on the gun when obtaining your grip and the Serpa seems to require that in order to function properly. At least mine do.
Bronson