No, the shims do not do anything except change how far the bolt has to go before it can vent air and return. Many people don't use any shims at all. If you did use shims, they would go in front of the carrier not between the carrier and the backing washer.
Originally Posted by chairChimp
It is quite possible that your on-off o-ring was pushed out of place or distorted due to sticking possibly caused by a piece of dirt. The result was air prematurely getting to the chamber before the sear was fully set. The higher input pressure allowed fast enough recharge to fire the marker. Once you reduced the input pressure, the recharge was not fast enough to charge the front chamber fro firing , but did prevent the bolt from resetting. By cleaning the valve, you effectively released any stuck o-rings and cleaned any dirt out of the system.
Dirt is one of the main causes of malfunctions in all mags. It doesn't take much so you should give your mag a good cleaning every once in a while jsut to make sure its operational when you need it.
Except for the Automag in front, its usually the man behind the equipment that counts.