I agree with Bakeoven Bill.
You need to enlarge the size of the vent hole.
Before you do this, take the unloaded pistol and insert a unbent paper clip thru the vent hole until it stops.
Then, use a flashlight to illuminate the inside of the bore. At the bottom of the bore, you should see the paper clip. If you do, we're on the right track.
If you don't see the paper clip sticking into the bottom of the bore, there is a possibility that it doesn't go all the way thru.
You need to find out if this is true and if so, why.
To find out if the vent hole does go thru the barrel wall, try pouring a little water down the barrels bore. It should flow freely out of the vent hole. If it doesn't, give the place you bought it from a call to see if they will replace it.
With a big bore pistol like the one you own, you can drill the vent hole up to 5/64 inches (.078) but before you do, try using a 1/16 inch drill bit (.0625) to enlarge it.
If you have a variable speed electric drill, use it at a very low speed and use almost no pressure to push the drill thru the hole.
In fact, because of its twist it will try to pull itself into the existing hole. Don't let it do that. Just use enough pressure (or negative pressure) as is needed to get the drill bit to go slowly in until it is completely thru the barrel wall.
If the gun still doesn't fire reliably after drilling the vent hole to 1/16", then redrill the hole with the larger 5/64 diameter drill bit.