If that gun has been sitting for any length of time without being used things start to dryrot on the inside...
Most people don't realize that a 12 gram co2 has a small amount of oil in it that helps keep the guts slick..and that is fine if you use the gun on a regular basis..but on the other hand all that oil eventually leaves some "sludgy" buildup on the innards once it starts to dry out and that sludge seems to like to eat away at the seals....