I suspect your powder. I bought some 777 pellets that did the same thing. Needless to say, I do not buy them anymore. You'd pull the trigger and hear a Poof and might see a fire ball show. But you're right.

Make sure that rifle is clean and that you blow the breech plug out with a couple primers before loading the first time. And when you swab, watch how wet them patches are. Some of them you purchase in a store are way too damp. I mist mine with alcohol or Windex and that's all they get. You can tell when you swab if the patch is too dry. If it is, give it a very little more, and try again.