Chicken not maturing

Got our chickens at the beginning of August, 4 light sussex from same breeder. Age then was approx 20 weeks.  Since then 3 have developed beautiful big and bright red combs and we are currently having 1-2 eggs a day.  However one is still the same as when we got her, very pink small comb and not appearing to mature at all.  All physically the same size and weight.  Just wondering if she is an oddball who wont ever develop a red comb and will be 'forever young', or whether she is just a lot younger and is playing catchup!