When I tried to hatch ducklings under a Buff Orpington a couple of years ago only one finally hatched (I think on day 29). After reading up on hatching duck eggs under hens, it sounded like the environment can be kind of dry and a little misting with a spray bottle every day can help the process (tho' I know this might be too late for you this time around). Still, as much as the misting advice sounds good, it seems I have heard of successful hatches without misting.