Great detailed response from Ridgerunner.
A simple way to narrow down the cause of the problem a bit would be to collect a good representative sample of your hatching eggs, and crack them open to look for the bullseyes on the yolks that show they've been fertilised. It would help a lot to know...
There's still a chance that some more of the eggs might hatch. The one time I had a broody hen, nothing hatched till day 25 or 26. So if she's still willing to sit on them, it might be worthwhile giving her a couple more days. If you're not sure what you're looking for when candling, have a read...