by no means put the good ones under 12/12, that will badly compromise yield and quality! The stubborn one is likely showing some "incomplete" auto expression, it's absolutely not a black and white thing, no off/on switch there,... We see many examples of this here all the time... sometimes it's just plain bad luck, or poor breeding,... you say this is Growers Choice? 
Figures! I'm seeing several misfires from their shaggy-ass genetics recently, no exaggeration.... I don't like to bag on breeders, UNLESS there's real empirical evidence of consistent problems, and IMO, now there is,.... you must be the 3rd or 4th person with this problem of lagging, slow blooming auto's of theirs that I alone have had to help with....
It seems to be showing photo period traits.. In that case, you can veg her under 18/6 for as long as you want to. If its a photo, then it won't flower until it gets 12-12 Regardless of what you do. Like Waira stated, we see it all the time with shotty genetics.. Your options are to either wait it out, or flip to 12/12.. If you wait until the current grow is over to try going 12/12, just be prepared for a large plant. If you isolate the plant for a week or so under 12-12, just to see if it flowers, it wouldn't stink much.. If it flowers, you can throw in back I. the 18-6 tent to reveg it and wait to flower it after your current run.. also, if it is a photo period, you can take a cut of it and run it again at a later time.
Those aren't buds forming. Those are female sex pistils. The plant is showing sex, but there are no buds forming in those pics. Photo period plants will show sex and throw pistils or sacs at around 6 weeks old.
