Those are caused by bad feeding and can be prevented by making sure the gun is lubed and bore-snaking them after every scenario.
A tip I picked up in the Simunitions Instructor school was to make you own inert Sims rounds. Simply get a fired case off the ground, force the "piston" back in to the case and put a bullet on top. This round will feed fine but will not fire, generating a click instead of a bang.
Something else worth doing, is to load make sure the round in the chamber is a different color than everything in the magazine. This allows you to evaluate whether they're getting first round hits or not.