Ok It's been a while but I figured out what was causing it (after having broke my firing pin). The A-Zoom snap caps have rubber where th pin hits. When this rubber is new and strong, the force of the firing pin pushes the bullet up against the chamber wall and you get a nice resonant metal on metal pop noise.
Well, when the snap caps start to wear out, the rubber starts to break and the firing pin is now just going into the rubber unobstructed. BEcause of this the frnt of the bullet is no longer forced against the front of the chamber and you just get a dead thud sound. But, of you cycle the snap cap, re feed it into the chamber, it will slightly move the area where the firing pin hits which will allow it make the pop noise when you pull the trigger.
Once the snap caps get a big enough hole in the rubber they never make the pop noise.
A solution Ive found is to take some cotton off of a cue-tip and stuff it into the crevasse with a pair of skinny tweezers. This makes the snap cap like new, and will make it really pop again.