Do you have them on Layer? At this point they probably should be. I don't know where you live, but are you getting a good amount of light each day? I've had hens that were due to start in fall but didn't lay until awhile late, I believe due to decreasing light.
I would guess they just aren't ready yet. I don't know what breed you have, but for example my Dominiques typically don't lay until between 6-7 months and my phoenix don't start until 8 months sometimes.
Yes, I've had them on layer since the end of July. I have 3 sex links and a cochin. My silkies, Mille Fleur and the random girl are younger. They are only about 4.5 months. I live in Missouri. It's always sunny here. I do have their coop in the shade though bc it gets really hot here.