First of all I would like to thank any one who is helping so far.
I don't think it is a matter of air circulation nor of having the lamp too close. The tallest plants are even closer to the lamp than the two plants with the problem, and are doing fine. Plus my hand doesn't get too hot when leaving it at plant height. I also don't want my plants to grow anymore vertically.
My ventilator is a moving one and quite strong. Heat buildup is inexistent.
The water drains perfectly out of the pots when I water them, and so it shouldn't be a problem of water.
I think roosterfish and green_nobody have hit the nail on its head: probably nute lockup!
As for the fertilisers, I am following the lower bounds indicated on Biocanna's website (i.e. if it says to use 30-40 ml of Bio Flores for weeks 4-last 2 weeks, I use 30 ml or slightly less).
Since I have to go water my plants now, I will use plain water with no fertilisers, and try adding extra potassium to the sick plants (thank you leafwrapper).
Haha I should probably save some money to buy a proper pH meter.
As you can see from the pics, I have had to use some yellow sticky paper to fight a fly invasion. Hope it's not THEM causing the probs.
I really hope it ain't root rot. I use about 270 ml of water for every pot every 2-3 days.