My girl is not laying

[knottydoll]

This is a picture of my only older hen called Amy. Two questions 1...What mix of chicken is she? She is mainly white with a small comb and she has grey legs. and 2...She is now 28 weeks old and no sign of any egg laying behaviour.
She is in a double run with 4 red sex links, all 15 weeks old.
Can any one help?!!!