It could be even though your incubator read "ok" it was actually a degree or so low. And they're still perfectly likely to hatch. That's the best case scenario.
Have you candled during incubation? Verified fertility or viability at lockdown?
I've got a mini temperature/ humidity tester in the incubator to read as well as so I can see the temperature is 37.5 and the humidity is 71% at the moment