There's actually a good chance that it's the propane canister. If it's near empty, the pressure can be lower before hitting equilibrium with the tank in the mag, meaning less gas goes in, which equates to fewer shots before running out.
I could be wrong, but try a new canister of propane and see if that fixes the problem. If not, well then I'm definitely wrong. But it couldn't hurt; Worst case scenario you get another tank to use and stock up for future games.
