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.
Hope this helps, and have fun! MW