The mother duck who started sitting on the nest (the slate blue runner duck) appears to have lost some of her interest in the eggs. Fortunately she has been replaced by the Rouen duck so the eggs are still being incubated. I've seen them sitting on the eggs together several times so it may be that the larger hen duck has crowded out the smaller hen duck. I suppose it is also possible that they have a schedule written on the wall inside their little shelter and its the other duck's turn this week. Personally, I'm suspicious that there may be a basis in fact for the Jemima PuddleDuck story by Beatrix Potter. I think hen ducks as a whole are somewhat less dedicated sitters that your average banty hen.
A few of the eggs have been culled by the ducks so we are down to 9 eggs in the nest. I think the ducks can tell when an egg isn't progressing as it should and its a good thing that they can eliminate the "bad" eggs. That would be like sitting on a ticking stink bomb. An incubated infertile egg will eventually explode. You wouldn't want to be around when that happens, much less sitting on it.