a small percentage of hens will never lay an egg, in commercial laying flocks its reckoned to average 1-3% its just one of those wonders of nature
Unless you have just a single bird or have them individually penned you may never even notice it , even within a small flock.
It can as already mentioned be due to an injury, deformity, disease or possibly even that the bird is not actually one sex or the other, it may look like a hen yet have no female (or indeed male) sex organs.
My parents have 6 Scots Grey hens, they get 5 eggs a day regularly but have never had 6.
I check them regularly and one of them is definitely never in lay, its the same one but they won't let me put it in a separate pen to test the theory