Sounds like a gas system problem to me. Get it back apart, make sure everything is there, pointed the right direction, and properly loosened or tightened as needed.
Not familiar with that system design in particular, but i've dealt with a few rifles that had parts that could be installed backwards or with small parts easily omitted or incorrectly indexed, would disrupt function that way.
Does the bolt cycle at all? Or just remain fully in-battery when fired?