@Rongzhong Li don't worry, your response was quick and I'm not in an hurry. Let me explain better the problem.
The shoulder fits correctly along. The problem is that neither the y1 nor the y1Pi can enter in the space between the ring and the neck servo (see picture, using the y1Pi) like in the tutorial or in the animation.
I can't push it, otherwise the ring will break (happened to one already, very sad about that).
Can you explain me if I'm doing anything wrong?
Thanks.