last egg

Hello, this is my first post here on BYC and I’m in need of some advice. To make a long story short I have a duck egg on almost day 28 that hadn’t popped internally. There was one other egg in the incubator with it but unfortunately didn’t make it last night and I don’t know the cause of death...

First time hatching... We have 11 mallard ducklings. So far 10 have hatched (started around 7am, ended around 2pm). The last egg had internally pipped (and was moving), but not externally. I made a "safety hole" in the air sac end since it had been 18hrs since the other eggs had pipped...