I think it has to do with the hammer strut blocking something or another.
So I pulled the trigger, pointed the gun down and tapped the back of the bolt towards the ground. Then the shaking was something like packing cigarettes. I tapped the front of the gun downwards onto some carpeted floor a few times. All of this to get the hammer to fall as forward as possible, in order to disengage the hammer strut from the mainspring housing.
I think.
The manual says the hammer should naturally fall forward if you point the gun downwards while pulling the trigger. Yeah right. Hopefully all this business won't last forever, as the weapon gets worn in.
The left-side thumb thing for magazine release is not so bad. The loaded chamber indicator is stupid, I think. I've always been under the understanding that all guns are loaded so you have to pull bolt back to see for yourself. Relying on a little plastic thing is... well. But my biggest gripe is the magazine disconnector safety. The magazine won't fall out easily, and I'm sure it adds to the trigger pull.