

Duplicate Stitch problems
I am doing a design in duplicate stitch for the first time and I am getting the same problem through all my tries.
Whenever I try to do the duplicate stitch design it is starting to bulge downwards a lot. It creates this Valley effect that I don't think will block out.
The base of sweater is made in Sandnes Kos, a blown yarn, and this sweater is recommended to knit in a blown yarn .
For the duplicate Stitch I have tried several different yarns I have at home. Everything from a heavier blown yarn to the combination I have more (lighter DK + mohair).
I have tried to do the stitches loosely, and tighter. In the picture it is tighter, but the result have looked the same whether they are looser or tighter.
If I use a much thinner yarn the green starts to poke through a lot which I don't think looks good either.
Is it a tension problem? Is it a yarn problem? A combination?