I agree with the layers theory. I have the same issue that my back curls get more weighed down and aren't defined. I wonder if maybe you are not getting enough product into the back. I decided to experiment myself with that and really focus on the back. When I did that I noticed more defined curls. So maybe the next time you add your products pay special attention to that area.
High Porosity, Fine, Thin Density, Low Elasticity