Waterfowl are harder to hatch out than chickens but I don't know why nor any secrets to share.
So many things factor in..how the eggs were handled, how old the eggs are and
a myriad of fluxuations with the incubation process. Stombergs has a great booklette on incubation.
Easiest and most sure-fire way to get a live hatch is to allow the goose to set on the eggs.
I haven't found any difference in peformance between my styrofoam Hovabator and my
Brinsea.
I keep the humidity lower than the other posters...around 30 - 35% for the first
25 days because if you have too much humidity the eggs won't always dry down enough.
I use a digital thermometer-hygrometer in my Hovabators which you can buy at
Wal-Mart, hardware
stores etc. The last 3 days I crank the humidity up to 80% using new sponges
(throw away after using) soaked as needed in warm water and have found this greatly
helps to increase hatching ease.
I use a separate incubator for hatching out incase I have eggs at different stages
in the same incubator.
Good luck!