So you're on day 17 of 12/12 right? They sure look small for being flowered. Don't get me wrong, they're beautiful, just small. I let mine veg for longer before I flip em.
I have always read that cannabis will not flower until it is sexually mature. Going to 12/12 before they are sexually mature just increases the amount of time needed to reach sexual maturity since they are getting 6 - 12 hours less light everyday. I veg mine until preflowers show, and then I flip em. Gauranteed full-on flowering about 1 week later if you do it this way. But a lot of peeps do it your way. I'm just saying you might be proving the validity of what I've always read if it's taking this long for your younguns to push pistils. They're probably just not sexually mature enough to flower/reproduce yet.