I didnt have that problem with my amby hammock, and DD was only 2.8kg when she first went in there.

You can buy a snuggle to help little babies stay on their back in the middle. Amby sells them. I think the directions for using the hammock say to place bubs in the middle of the base with their hips underneath the frame at the top of the sling.

Do you have the head end raised too much?

I wrapped DD in her hammock till she came out of it at 7months. (wrapped her for another month or so after that too)