These plants could have top colas at least two inches wide and 8 inches tall by now. besides the fert, you should flower with a HPS bulb for the red spectrum. if you get it right, and have your tops the right distance from the lights, you should get a decent harvest still. next time try pruning to get more tops and to make all the tops close to the same height. some plants like to grow a bit lankier than others and it looks like you have more than one strain, so pruning and bending are probably necessary.