Most mechanicals will close if they pass thru the target and the arrow stops. Like if you are shooting at a piece of foam your arrow will pass partially thru. The de-acceleration of the arrow is enough to close the blades again.

Try putting a piece of paper or thin cardboard on the back of the target, heck even the front should work. Then look at the cuts in the paper to see if the blades did in fact open. I have rarely seen a mechanical fail to open on a target. And very rarely heard of them failing to open on game. The biggest complaint I hear is that they don't penitrate enough. This would simply be that you picked the wrong head for your set up. Some require more energy than others.

Paul