It happened the very same thing to me, one out of two Gelato Auto didn't start flo at all. I've tried to put her to under 12/12 and she did flower, to reveg as soon as I put her again under 18, so I came to the conclusion she was photoperiodic and dump her, since I didn't want to put all 4 of them under 12/12.
I hope is not your case, would be super weird if all of them are like this.
4F1M6 could be right maybe is a longer veg too.
Best of luck with them!