I didn't say anything about it before as I have no idea. I have hatched 100s and 100s of chicks so you'd think I'd be good at candling and I'm not. I could swear I've seen some of mine do that too and hatched.
Normally, the air cell is up at the big end, but if it's over to the side a bit, I...
There isn't anything you can do but wait. This isn't so unusual yet. Your incubator could have been say .5 degrees low the whole 21 days which would delay them slightly.
Just an idea that before you set eggs again, get yourself a trustworthy thermometer/hygrometer. There are cheap ones...