I agree with previous respondents: try feet higher, with more sag in the hammock, and lay diagonally across the centerline.

The way it was explained to me was: think of your body as a long triangle. Your shoulders are the base, and your feet the pointy end. if you put that big triangle on a slope, it will slide downward if you put it pointy-end down. If you raise the pointy end, it's not going to slide.

Works for me. Good luck, and congrats on your great Christmas gifts!