I'm willing to bet it's a maturity thing. Often, a plant that is not yet ready for flowering will delay, even under the 12/12 lighting. Perhaps a tad longer in the growth stage next time would be a benefit.

No harm, no foul, but most strains will perform better if you let them stay under 18/6 till they show pre-flowers. (calyx's and pistils) For some smaller grow rooms or those flowering under CFL's and for some sativa dominate strains, this might be an issue though.

Off topic:
Speaking of the light versus darkness...did you know that light bulbs don't emit light...they suck the ambient darkness out of the room...? (dark suckers) Once a bulb has sucked all the darkness it can, the bulb burnes-out leaving soime of the residual darkness it has caught, as a dark spot on the inside of the bulb. (an old physics theory...a result of trying to determine the speed of dark)