How are you giving them complete darkness and guaranteeing that it comes at the same time every day?
I see you mention sunlight. I assume you must manually move your plants or put down curtains to give them darkness. I also assume that you are not able to do this at the same time every day.
I suggest you look at all the grow logs on here and notice that every successful indoor grower uses timers and a grow chamber that may be darkened fully just by switching off the lights.
What does your darkening area look like?