I would leave the alone for another 2 or 3 days. We just had our first hatch and it started on day 23 and continued one egg a day for the next four days. So as someone so wisely told me on this thread....Be patient it will happen!(I've mostly posted at the Washingtonian's board lately).
I've had nine SSH eggs under a big old Australorp hen for 23 days now; I tried to candle a couple of eggs yesterday using the sun-peephole method but I have never been able to see anything in an egg, ever, starting back when I was a little kid and we bought uncandled eggs cheap and Mom candled them to sell in our store and tried to teach me the skill, back in the fifties. It's a shameful admission, I know. I last tried with a bunch of EE eggs I incubator-hatched last fall: couldn't see anything, ever, and ended up with 14 chicks.
How long do I wait to replace the eggs or try to break her brood? They should be fertile- all the ones I break to cook have well-marked bullseyes and the hens have dirty foot-prints all over their pretty backs- but I don't know for sure that she didn't take too much time off the nest early on. Now she won't leave the nest at all! She's raised at least one hatching before, according to the fellow BYCer I got her from.