FWIW - I run into the same thing with some White Widow autoflowers I'm growing. I was coming up on 10 weeks from the time the sprouts first broke through the soil and didn't even have any signs of pistils starting to develop. I had been running lights 20/4 from the beginning. The plants were healthy, bushy, loaded with future bud sites, and gave zero indication that they were ever going to flower. So at about 9 1/2 weeks I cut their light to 12/12. Within 3 days I had pistils starting and within a week was well into flower. I've kept them at 12/12 and they seen to be progressing nicely.
It could have been a coincidence, but I'm leaning toward thinking cutting the lights gave them the nudge they needed to start flower.
Good luck with your grow!
Butch