Duckling low hatch rate with dead ducklings

[pinksapphire]

I set 15 runner duck eggs in my bransia maxi eco, water was kept topped up and they were tuned three times a day.
Two eggs were infertile
Five eggs stopped developing around half way through
Four baby ducklings successfully hatched on day 28-29
Today day 31 I opened the remaining four eggs to find fully formed dead ducklings
Any idea what went wrong, do they look shrink wrapped?
And what would be the most likely cause for four to stop developing half way through?
I have hatched chicken and quail eggs in the same incubator with very good hatch rates

 

[deer trail duck farms]

do you have a separate thermometer and hygrometer in the incubator to verify temp is right and humidity is correct? my guess is it was way to humid. i have a nuture rite. i only add water in the morning and just a cap full is enough to last 24 hours till the humidity goes back down. it doesnt take much water at all. to much humidity is essientally drowning the ducklings.