you should of probably of started flowering 3-4 weeks ago as your plants are going to get a lot larger and you dont really have the room or enough light to be flowering large plants.
you might consider topping and removing alot of limbs or pruning just so you can even up the canopy and have a little more control of the growth that you will get during flowering.
generaly you will not start feeding flowering nutrients until 2-3 weeks into 12/12 .