I suppose it's possible that the shells were too hard (perhaps the jenny had too much calcium in her diet), but my ducks (especially the Pekins) have extremely hard shells and have always hatched (100% hatch rate on my ducks), but they take longer from internal pip to external pip and zipping also seems to take a long time. Often I get more than 12 hours between internal pip and external pip, and another 12 hours from external pip to zip.
I just had a 100% hatch rate with my turkeys with humidity throughout incubation at about 50% in a smallish incubator (24 egg capacity). This being said, the first turkey pipped and zipped at day 27 and the last one pipped and zipped 36 hours later.
I would think that if the shells were too hard you would have at least had internal pip, but if you didn't get internal pip and the eggtopsy showed developed chicks you might have given up too early. I blame my patience (or lack thereof) as the cause of death for a couple chicks in past hatches; I believe that had I waited I would have a couple more chickens now than I have.