Don't take offence, but some questions.
1. How do you know the paintball tanks are full? Have noticed the guns shoot poorly when the co2 tanks get low (still have 1 or 1 1/2oz. of co2 in them). Cool weather increases co2 use, and tank o-rings can leak, so if you're judging by the number of shots it might be that they aren't as full as you think they are. Need a scale. get the EMPTY weight of each tank, and can figure the co2 they contain (about 14.5- 15.7oz. for a 9oz, depending on make/pin valve/etc.).
2. There is enough room for a pellet to fall down between the bolt and receiver, into the bottom tube. If it does, it gets smashed ahead of the hammer, and if there is will produce erratic shooting or even no-shooting at all (it blocks the hammer from striking the valve stem).
3. The valve stem can become bent (and it won't open when struck) or the seal can detach from the stem.
4. Paintball tanks pin-valves can refuse to open, feeding no gas to the gun, if the pin valve on the tank is damaged or if there is some stray "funk" keeping them from screwing on tightly.
