So many reasons why that could happen. If I counted right, you are only at day 21?
First, wait longer. It sounds like your incubator may have hot and cold spots which is causing a staggered hatch.
If by day 23, nothing has happened, then they are likely duds. Candle the eggs to see if there are any developed chicks.
Then if you see no movement, no signs of pipping, after day 23, I carefully shake the egg to see if there might be a live chick. Then I gently open the shell up. I generally find dead fully formed chicks, often in various stages of growth indicating death on different days. Sometimes I find a gaseous, gross, green ball of sludge, so you'll want to open the egg inside a zip bag to prevent explosion of really gross stuff on you.
The most precarious time for chicks is the first 3 days and the last 3 days.
The first 3, the embryo may start, but stop.
The last 3 days, the chick may not be in the right position and can't hatch. Or, the humidity is too high which forms too much moisture in the egg, and the chick drowns. Or, the chick was simply not strong enough to hatch. Or, there was internal problems that prevented the chick from being able to live.
Those that don't hatch have a reason they don't hatch. If you are artificially incubating, you should note if there was any abrupt changes in temperature or humidity at the end, and double check your temps in several places.
Lofmc
