Because the plant cannot uptake enough nutrients to meet the lighting demands driving photosynthesis... so you either need to foliar which I don't recommend or reduce the light and thus the photosynthetic rates and demand for nutrients.
Or keep light levels up and watch the clones try to shuffle mobile nutrients around and become unhealthy... not a good option either
Ok my last post was about clones I got my threads mixed up.
Yes same thing though... over or underwatering will affect nutrient uptake and you see it try to shuffle N and other moble nutes to new growth... the immobile nutrients will show deficiencies in the newer growth and mobile in the old growth.