My plants die ?

As 001* said, it could be a nitrogen deficiency For me it's a little strange, because I use the same soil, and if you add something to it for the growing phase there shouldn't be a lack of nitrogen while they're still at that size. From what I see, larger plants don't show signs of a lack of nitrogen, so smaller plants shouldn't lack either. I've had similar situations and in my case I think it was because there were too many nutrients in the soil at a very early stage

nitrogen deficiency. if in soil, the repotting should help as the new soil has a fresh charge of nutes, but you'll want more nitrogen for those plants than before if it's the same soil and same fertilzier. Try to up N proportion of your mix without affecting the level of other nutes as best you can and move forward. stop thinking in terms of "they were all treated the same" - there are thousands of things that could be different including genetics. A solid ratio of nutes should work well for 95-99% of plants, but it may take some time to work it out -- especially in soil as youa re learning to supplement what the soil lacks over time through trial and error. Take notes, do better next time, and evenetualyl you can treat the plants all the same and most of the time that will work well. biobizz light mix? is that coco with a light nutrient charge? if so, soilless is a much easier thing to control and hit the ground running. try these ratios of nutes: PPM 120-130 N 40-60 P 180ish K 100+ Ca 75ish Mg 100+ S Some of that can vary due to the water that comes out of your tap. that is a very light concentration at ~1.3EC. This is calculated from guaranteed analysis labels not using some shitty 5 USD TDS pen. Along with tap water, it'll be a hgher EC in reality. In flower near end of last vege growth (stem elongation / leaf production) reduce N to 110 ppm give or take. This ratio of nutes, which may need a difference overall concentration given local variables, will work on 95-99% of plants in a soilless or hydro context. It maintains a critical level of nutes around the roots in a way that doesn't lock out anything over the long term.