Why won't my Muscovy hens care for their babies?

I have 4 wonderful Muscovy hens I got from a member, they are a year old so pretty young.  In the spring all 4 sat on nests wonderfully however once the ducklings started to hatch they didn't look after them, I found one dead and one almost dead.  So I took all the eggs and put them in the bator.  I figured it was because it was their first time and they didn't know what they were doing.  
So they were all sitting again, they seem to love sitting on eggs.  I went in the coop this morning and there was a duckling laying in the middle of the floor almost dead and very cold.  Stuck it in my bra and headed to the house to plug in the bator, put the duckling in my bra under a heat light.  Went back out to finish chores then checked the duck coop again and there was another duckling in the middle of the coop shivering.  So I took 20 eggs to put in the bator and left a few for the hens, thinking maybe there is just too many for them... Oh both ducklings have survived and are doing great.
Why aren't they looking after the ducklings?  After the eggs hatch in the bator should I put the babies back with the hens?  I just don't know why they aren't looking after them and I don't know what to do.

How many drakes do you have? Also where are they nesting?
I have one hen that always hatches over 2 dozen ducklings, way to many to handle in our wide open yard so I always take half away and she does a good job with the rest.
But I have another hen that always nests up on the shelf of the goose pen (used to be a shop so has a nice shelf), well since the ducklings don't all hatch at once they come out of the nest and onto the floor, and so momma has to make a choice of staying with the eggs still to hatch or to go with the 1 duckling. That might be an issue with your ducks. Have they ever hatched out completely and then not taken care of the ducklings?

These four hens are in a coop with a large fenced yard, there is one drake in with them. They have all nested beside each other on the floor. I removed the 20 eggs and left some with them. When I checked earlier I seen 4 more ducklings, I left them in the coop to see what happens, put a water jug and duck starter in the coop...will check every hour to make sure no duckling dies.

Maybe They just need some time to figure things out