Droopy leaves, thin base of stem??

Hi all,

I am getting random plants at random stages that seem to be going well, stop growing then the leaves go droopy and eventually die. I have also noticed on the affected ones that the stem at the base is really thin. I'm not under watering them and i flushed one thinking it might have been a nute problem and seemed to make it worse. Could I be over watering? Temps are about 30 at day and 18 at night.

Any help really appriciated

Hey and welcome to RIU
I'd love to help but it's hard without pics.
Also im in the US and I do my temps in F not C so I dunno about your temps.
How old are they how much nutes are you using? very little info, id love to help
check out these threads
https://www.rollitup.org/newbie-central/1079-before-you-post.html
https://www.rollitup.org/marijuana-plant-problems/157345-have-plant-problem-check-here.html
https://www.rollitup.org/marijuana-plant-problems/38772-guide-diagnosing-plant-problems.html