Has the duck that is half out absorbed all of its veins? and is just stuck to the membrane? If it comes out and detatches from it's membrane too early it could bleed to death. I just uploaded these pics of one of my most horrendous hatches ive ever had, 1 out of six, but she survived and is a lovely duck now!!
http://i1184.photobucket.com/albums/z328/Kayla_Lewis/IMG00453-20101210-1932.jpg
As you can see, she started to hatch and was stuck in the shell for about 2 days and i thought enough is enough. I peeled the shell off the membrane as she was trying to move around now and again so that the weight of the shell didnt detactch her umbilical cord too early, eventually she just wriggled away from the membran. I wouldnt recommend this on ducks hatching early as this duck had pipped and not hatched afer 4 days. This is just to prove to you, there is hope.
And i shouldnt imagine they are suffering. It's only what they do fromm inside the shell, break away all the tiny blood vessels bit by bit, these guys are just doing it early. Its just a waiting game, and a game of testing your paitence- which is tough!! I hate it when i have a hatch i send myself out for the day!!