Hatching worries, day 21

Have just had a call from a freind, She has 12 eggs due to hatch today, Said she could hear them chirping in their shells yesterday but noting since last night.
I've never hatched so have no idea, Im sure some of you will though.
Is this normal or have they died in their shells?

It's so hard to have to wait but be patience is key!!

I sometimes put a tea towel over the viewing panel of my incubator (not the airwents!) so I'm not tempted to interfere with the hatching process!