Try removing the non-return seal from the inside of the cap and blow through the valve from the outside. There should be no resistance. If you get resistance then the valve may be blocked, mashing you aren't getting much (or any) co2 into the barrel. Another symptom of this is can be that the outside of the co2 canister nozzle frosts up while injecting due to the co2 venting past the black o-ring to the outside air rather than going into the barrel.