I think it is a matter of genes and hormones, but you can try putting an "egg" in her favorite nest and see if she'll take the hint. A lot of people use wooden eggs as they seem to mimic the weight the best and hold temperature pretty well if the hen chooses to sit on it. But in my experience, there are some hens that will and some that won't and some that give it a shot but can't seem to ever achieve the "trance". Silkies are almost 100% in the broody category where some of the hatchery production hens are at the other end of the spectrum.