When testing to determine if you need to add shims, they will only help if the bolt moves forward but does not vent air. If the gun fires but does not reset, or if the bolt chuffs but won't reset, or if the bolt moves forward and leaks air out the front but won't reset, then shims will not help. Shims only shorten the distance that the bolt needs to move in order to vent air so that it can reset. Most of the time, the bolt can move forward far enough on its own, even with a breach blockage, to vent and reset.
Except for the Automag in front, its usually the man behind the equipment that counts.