I don't think there's anything you can do this late. What was your humidity at for the first 18 days? I keep mine at 35-45% so they lose enough moisture so the air cells are large enough. I had an egg that it's air cell wouldn't get bigger. All the others' were fine, and one was possibly too big. And I couldn't figure out for the life of me why it was so small. It wasn't as small as yours, but it was causing concern for me. (I'm new to hatching. This was my second hatch. Put 8 eggs in, 1 non-fertile, one quitter at day 12, and 6 chicks that hatched beautifully) But that little guy popped out just fine. The thing you need to worry about is, if there is too much moisture in the egg, when they internally pip the fluid will gush into the air cell and drown them. I can try to add a pic of the egg so you can see what size the air cell was on mine(it was a bantam egg just to let you know). I hope they make it. Good luck.
These were taken on day 18. It had already internally pipped when I was candling for the last time. I was so happy when I saw it had externally pipped because that meant it hadn't drown. Anyways, here are the pictures.
And once again this was a bantam egg. And sorry the pictures are so bad....
Thank you and they were in a incubator with a faulty hydrometer for the first 14 days so humidity was to high. Moved to new one and have been keeping them around 50% humidity. Should have gone lower. However from day 17 (when I realised air bubbles were too small) to now (end of day 19) the airbus led have got much bigger. They not look around the size of your egg^^ .So happy, hopefully they will make it now, although one of the air bubbles is very very large and detached (has always been like this) is that guru teed death? Here are some pictures when I candled just now.