X-Muscle neck question

Hi folks,

Anyone using the X-Muscle addon?

Something that’s puzzling me is that the neck seems to be fixed in a straight line without any curvature possible. I’m sure it must be possible but i can’t work out where i’m going wrong.

Please check out the video below. First you see me move the only neck controller. but it doesn’t create any curvature in the neck.
Then I indicate the two bones that i can’t manage to bend and then i put an X where the bend needs to be.

Any ideas?

Thanks!