Sure they do but 4 zips from a plant that size (5 month veg) is not normal! Unless you took clones and vegged the clones for 30 days then yes, 3-6 zips under a 1kw hps is about right assuming your plants were healthy and fed on time. There could be plenty of things that caused your low yield. Not enough fresh air, old bulb or defective ballast/bulb,dirty hood,cheap hoods,hard water problems,inaccurate pH testing,under-developed root zone ect... I would say genetics but you've tried different ones from a completely different mom/dad right? I want to help, not trying to sound like a dick but i can pull 2-3 zips from one plant (30 day veg) 300w spiral cfl's in a 3 gallon with regular ass ocean forest, that number will jump under an hps and there are plenty of growers here that pull 25-50% more then me! Post your journal link up here if you make one.