Hi Jenny
Once the female swan has most of her eggs hatch, she will take the cygnets to the water 24 hrs after hatching. She will return to the nest for a couple of days. During this time, there may be some eggs not hatched and late hatchlings. However, if there are late hatchlings, the mother swan will not wait for the them to catch up to the rest of the family. Usually, these are the weaker runts and nature will not allow the rest of the family/ flock to be subjected to illness or slower cygnets which also invite predators.
Better to lose one or two than the majority.
Now, if a predator or other disturbance was in the area, the mother would quickly move the viable cygnets to a safer area. The Regal Swan