The I-12 is an inertia gun just like a Benelli M2 or SBE2. It is not a gas gun. These inertia bolts have a rotating bolt head. This bolt head must rotate itself into the locking lugs machined into the barrel extension like a bolt action rifle. If you let the bolt down slowly it will not fully rotate and lock up and you will not get the primer to ignite because the firing pin will not reach the shell. You MUST let an inertia bolt slam forward with the full pressure of the action return spring slamming it home.
If you are letting the bolt slam forward then check inside the locking lugs to make sure there is no debris preventing the bolt head from fully rotating and locking up.