Yep, bubba is right on. I have heard of others getting 777 pellets to fire, but personally I have never had any success with the factory breech plugs utilizing the plastic primer carriers. Just too much flame can escape and not maket it into the charge, causing the hang-fires. Pyrodex pellets on the other hand work fine, as they have a touch of real Black Powder bonded to the end to aid in ignition.
If you feel you must shoot 777 pellets, witch are actually the most expensive BP Substitute on the market next to White Hots, you will need a better breech plug design.
You didn't say what brand/type of primer you are using? The 209 primers designed for muzzleloaders only, are going to be WAY too weak. You might try Federal 209A's or CCI 209M (Magnum) primers, but you would probably need to use a brand new primer carrier each time as well. The primer carriers wash out from the blowback and can loose .005" to .010" in just a few shots. These can run from $15-$18 per 10 pieces, at $1.80 per shot, if a new carrier were used each shot.
I would just ditch the 777 pellets for loose powder, much less expensive, AND your rifle goes bang when it should.