First....you are lucky not to have a case burst on you. If you have never experienced that you are lucky! Its not pretty or fun.
What loads are you using?
What buffer and spring did you use in the rifle?
Have you tried dropping a new factory round in the chamber and checked for it sitting at the right depth?
Does your bolt easy move completely forward and sit nice and firm against the breech?
As mentioned, the problem is you have too much pressure in the casing with the bolt not fully in battery. The rear of the case is not supported and you have pressure expanding the case. If you do this on a case that is weak, it will blow out. Been there and done that and its really not pretty.
A blow-back firearm seems overly simple at first glance but the truth is while the parts are simple, the physics of what happen are a bit more complex. The mass of bolt, stiffness of the recoil spring, weight of the bullet and speed of the pressure rise in the chamber all come into play. Call it "timing" if you will. They all have to work together perfectly.