Obvious suggestion...check your air pressure. Also check for a sluggish solenoid valve; if the valve opens slowly the air creeps in and the knockout piston does not snap down quickly, the impact is needed for the knockout. If the piston moves slowly at first it takes up the clearance and then just pushes instead of knocking.
I had this problem on a smaller machine and loosened up the solenoid valve by disconnecting the air line leading to the valve and squirting in a bit of transmission fluid.
You use the remedy at your own risk!