Did you open after day 21 to help this one? If you open it during lock-down it affects the humidity, which can affect your hatch rate %.
I am no pro but I have a high % hatch. This is what I do. I Never open mine during lock-down for Nothing. I open it on day 22 unless all the egg have hatched...
Humidity, possible! How often did you open the incubator----during lock-down? I know you opened it to help this chick----was this after day 21 or earlier?