Do they look mature?
Do they have good combs?
I think chickens develope at different rates.
My 3 black rocks came to lay quite quick, without fuss and have pretty much laid all year round for over a year now. I have 2 brown chickens, 1 only lays spasmodically, the other has never developed a proper comb and is rather fat and lazy, and I think has laid maybe 2 eggs if that. Dad says I should eat her but I can't cook 'Penny Chicken'!!
Be patient if they are happy and content they should lay.
Good luck