Some poly battings are softer than others. Hobbs Polydown is very soft but has a lot of loft to it. Cotton batting softens with every washing. Warm and Natural has an especially stiff drape. My recommendation would be to try Hobbs 80/20. It has overall great properties, loft but not too much loft, softness, a high cotton content.
Aside from the batting, what kind of thread are you using? Cotton thread, especially 50/3 will stiffen up a quilt. Polyester thread does not, even when close quilted, even a 40 weight.