I am not near a computer just now, but there have been a lot of walk discussions recently in the last week or so in the forums - some of them may be worth browsing as well. I found the position of the handles can make a difference - e.g. the distance between the “Head” and “Neck” handles. But I am not a walk behavior expert.
I think the problem is you don't have the head attached to the body correctly, and you don't have Head and Neck tags set correctly.
First, the head is independent (good), but you need to drag the handle so it overlaps the body and "attaches" to the body. The handle should turn green when this is right. (See the body turns green when the head origin is over the body, linking the head to the body)
Next, I put handles on the torso for "Neck" (I removed your other Neck tags), then I put a handle on the head above the neck. Once I did that slump worked fine.
The above image shows the "Head" handle I put in place. The shot below shows the Neck handle on the torso layer. (You had "Neck" as a part of the head - which is logical, but I don't think is the right way to to it in CH. I have had more luck with the Neck being part of the torso myself - feel free to experiment some more here! There might be more than one way to get it going.)
Just to expand on the Neck, what you are doing may work fine - but my default these days is to make the neck artwork a part of the Torso, then attach the head to the neck. In your artwork, the neck is a part of the head artwork layer. I *think* the Neck tag needs to be a part of the torso for things to move best - but feel free to experiment. I tend to get one thing going then repeat it each time - I have not tried all the variations possible.