I had 4 eggs in the incubator, all were fertile and growing. Last I checked all were alive a week ago. Hatch day was yesterday and we had 2 hatch around 6am and 10am. 2 remaining eggs were checked today and one obviously stopped developing a while ago since I can still see through most of the egg. The other is solid through out but I'm not sure how long to wait.
Do you see any movement in the egg when you candle it? If not, then I would open up the egg and see how the chick is. Start at the air cell, and work your way down. Don't tear off too much of the egg at once, or you'll risk ripping blood vessels and harming the chick, if it is still alive. If you determine that the chick is still alive, stop peeling the shell away and put it back in the incubator.
I had a rotten first hatch as I didn't calibrate my new thermometer and unbeknownst to me it was 6/7 degrees off so the actual temp was less than it was showing. The one little chick (out of two) that hatched and lived did not hatch until day 24. A good share of what I have read says give them until at least 25 days. she a healthy (loud) and loving chick who turned 4 weeks old today. I wouldn't chance opening eggs until 25 days, you could always have some late bloomers. My opinion.