anyone who knows about preflower cycles?

I have done 12 crops all from clone. never used seeds. this time i used seeds. all genetics and feminised. i never have any problems growing and i didnt this time either untill i turned the light cycle from 18/6 to 12/12. i have 12 plants and they veged out for 6 weeks after two weeks of seedling time under a dome. 2 have preflowered so i figured the other are just around the corner but it has been 2 weeks and the othere have not began to flower. i know that u cannot flower a plant before it preflowers but could i have caused any stress on them putting them on a flowering cycle before their preflower. or is this a common thing to do and it just takes time. usualy from clone u have many pistols showing by two weeks. ya know.

i have to note that everything is perfect in the room and i know what i am doing on the grows just like i was saying never used seeds before and dont know about their preflower cycles. so please let me know even if u have planted a seed and flowered it before it showed signs of preflowering.