My nesting boxes are set up so that they'd have a hard time kicking it out. How are your nesting boxes set up? Perhaps your gals have decided their pampered behinds prefer a softer material to lay in?
Straw is definitely more rough to sit on than hay, but a thick layer of shredded paper is even more cushy.
I use a combination of hay and shredded paper, because I notice my hens end up pushing all the hay up the sides of the nesting box, exposing the bottom. Adding shredded paper has definitely helped with that and they seem to like it better.