Chickens have been known to be great hatchers of duck eggs. But the ones under the heat lamp may not be doing well. There has to be a certain amount of humidity and a specific temperature for eggs to develop properly. You might want to candle them, and also the eggs under the chicken, to check for viability and remove any that are duds. You could try posting in this other forum for some better help:Mine are fat brats. I ended up with a boy and a girl to each breed. My husband's picks... Go figure They are almost a year old and lay fertilized eggs, but they do not tend to the nest at all. They just want to eat and play in the water. I have two eggs at the moment I am attempting to hatch under a heat lamp. Not sure if it will work, but it's worth trying. I have one chicken, Catwoman, laying on six duck eggs most of the time and is very possive with them. Maybe we will get lucky and have more little ducks.
https://www.backyardchickens.com/forums/incubating-hatching-eggs.5/