How is this occurring?
If you're merely dry firing, then you will need to bump the butt on the ground, slap it hard with your hand, or just switch the barrel selector. Inertia guns need recoil to reset the trigger.
If you're using good ammo and there's and issue, it could be the firing pin is slightly worn, and coupled with possibly some deep primers, isn't making contact.
A little more information would really help.