What breed is your female? If it was a muscovy male that bred with her, yes, it could take the offspring longer to hatch. It would also mean the offspring will be mule ducks and infertile.
If you started them on the 20th, the hatch date would have been yesterday. Have they started to hatch yet? What has your temperature been? And the humidity? Is your incubator still air or forced air with a fan?