Larger breeds can be slower to hatch and cold weather can slow down development.
Have you candled at any point? Were they all alive going into day 18? You could take them off one at a time to candle, just make sure to keep the egg's orientation intact so as not to confuse them if they are moving into hatch position.
Are you counting correctly? The day you give your hen the eggs is day 0. It does not count as day 1 until 24 hours after they were started.