I seem to never be able to give enough N to my plants either, particularly with Sat doms. I feel a contributory factor to my issue was actually light bleaching, which I try to address as best I can. I also stepped up my pot size from 10 litre to 20 litre as I thought it may have been lack of root space which was leading to under developed root mass and therefore not optimum nutrient uptake. Seemed to work with plants vegged for the same time (4 weeks) - the two hybrid phenos (Sugar Punch) stayed very healthy green throughout, with the Sativa being the only one to show yellowing (but she was an explosive new growth when flipped to flower pheno). I have a chance to offer comparison as I now have some 12/14 week veg plants flowering in the same pots. These longer veg plants are showing varying degrees of yellowing now they're flowering. This leads me to believe the pot size does contribute.
Oh and I also got some 'Bloom Cal/Mag' to use in combination with the Bio Bizz 'Grow' which was my veg nutes. The calcium and magnesium are reported as aiding in the uptake of nutrients by the plant, and it is solubilised with nitrogen which seems to improve the whole processing of the Nitrogen. My plants have certainly improved since I implemented all these changes. (but like I said, the 14 week veg plant is still yellowing, it's an ongoing process)