More often than not your overwatering. Male sure to let the soil completely dry out the plant will go thru a cycle where it fist gets real perky and good looking when soil 1st dries out. When the ppant start to show signs of being to dry after the days of it looking great all day then you want to water the plant right b4 lights out. So I normally don't check what the plants look like the 1st 6 hrs of lights on because alot of plants will still have a sag to them from night time. So around hrs 7 to 10 from lights on if the plants are looking like there starting to not perk up where they have been then wait till that night at lights out.
If your not over watering then maybe the light is 2 close and it's starting get stressed
is it on all branches or one ? if one , have a read :
https://www.lahuertagrowshop.com/blog/en/how-to-prevent-and-get-rid-of-fusarium-in-cannabis/
no diary, hard to help ..