I've had ISA browns for a couple of years, and in that time none of them went broody. It's a long shot, but I found that putting golf balls in my nest boxes helped make my broody hens brood longer (like three months compared the the usual four or five weeks). Maybe your hens will feel like going broody if there is always an egg (err. . . golf ball) in the nest box.
If you really want a broody hen, though, I've heard having a silkie around is always a good option, though I've found that my Easter Eggers have gone broody 2 or 3 times a year.