Doesn't air up as in the 12 gram doesn't get punctured or it punctured but all the co2 just rushes out? If it's not getting punctured it's the piercing pin that's blunt / missing, if the airs rushing down the barrel try cocking it before putting the 12 gram in. If it still leaks down the barrel then it's probably the cup seal on the valve stem but you might get away with stripping it down and oiling it.cool, I put a 12g in and it doesn't air up so now I can go and hunt some parts down.
cheers tom