What do I do if my ducks are taking too lomg to hatch but are ?

Am I currently at day 31 of incubation, with my khaki campbell ducks, (I heard they only take 28?) and I am getting worried and wonderring what is wrong, because one is every few hours, but not hatching, or even making a crack! -What do I do? My humidity is correct, the temperatue is perfect, I stopped turning them when suposed to, and still, they are NOT hatching! What could be wrong?