my ducklings are late hatching

[lillypony]

hi I am hatching my first bundle of duck eggs and so far I have had 1 infertile 1 stolen and 1 dead whilst hatching. I have got 2 different breeds but I only know what 1 of my breeds is. I had 1 chick hatch on day 28 and it is doing fine at the moment. but it is day 31 and my runner duckling hasn't broken the air sack yet. I have tried wrapping it in a warm damp cloth because they are under a chicken but I don't know if it has worked yet. is it normal for it to be this late? if so when will it hatch? and is it okay?
thanks if you reply

 

[iluvmybabiez]

Have you tried candling them? I've had a better success rate when I've used an incubator.