After the gun has been disassembled and reassembled several dozen times, it gets looser. Files should be applied with caution.
My opinion is that the sear spring is installed wrong, but without the gun in hand I would not swear to it. If the sear spring gets ahead of the sear, the sear will not engage the hammer. If the hammer is cocking but not staying cocked and follows the bolt, I almost always think it's the sear.