Have you opened the unhatched eggs to see what you think may be happening? These links may help with that. As you can see, there are several different things that might be the problem.
Mississippi State Incubation Troubleshooting
Illinois Incubation troubleshooting
I suggest you check the accuracy of the thermometer and hygrometer. I know Brinsea make good incubators but I don’t trust any temperature or humidity settings or readings until I’ve confirmed them.
I’m not familiar with that model, but where are the vents? Are they open? With a force air it should not be the problem but late in incubation especially the chicks need fresh air to breathe. They can suffocate. Usually that’s more likely to be a problem with a still air but I’ll mention it anyway.
Something else that is not your problem. If you try to hatch the eggs upside down (pointy ends up) the chick often cannot find the air cell so it drowns when it tries to internal pip. If the eggs are not turned early in incubation you can also have problems. But in that model the eggs lay down and are automatically turned. Like I said, not your problem.
I’d question the humidity during incubation. If the humidity is too low during incubation, the chicks can be shrink-wrapped even before they external pip. Raising humidity during lockdown won’t help with that. If the humidity during incubation is too high, the air cell does not grow large enough so the chick drowns before it can external pip. The chicks with high humidity during the incubation portion tend to be large and mushy also. Sometimes they can’t move in the egg to position themselves for internal pip. If you haven’t confirmed that the humidity reading is at least close to accurate, I’d think this is a possibility. Opening the unhatched eggs can really help you diagnose this, even if you don’t check your hygrometer or humidity reading.
This one kind of concerns me. Are you being too impatient? Are you waiting past 21 days before you discard the eggs? I can’t tell that from your post. The 21 day thing is just a general guideline. It’s not all that unusual for eggs to pip and hatch a few days before or after 21 days of incubation. There are a lot of different reasons for that, but a really big one is average incubation temperature. If your average incubation temperature is low the eggs can pip and hatch late. Check the temperature reading.
One thing that sort of goes along with that, are you counting the day s correctly. An egg does not have a day’s worth of incubation two seconds or two hours after it goes into the incubator. It takes 24 hours for an egg to have a day’s worth of incubation. If you are counting days, you say “one” 24 hours after you put them in. A good way to check this is the day of the week you put them in is the day of the week the 21 days is up. If you start them on a Friday, the 21 days are up on a Friday.
That’s all I can think of off the top of my head. You may find something else in those links. If all else fails, call Brinsea and chat with them. They’ve probably answered your question before for someone else.