What can cause this problem

They are hungry. Dry soil amendmants are a poor choice for cannabis, they rely on wet conditions to become activated and cannabis does not like constant wet conditions. Liquid, fast acting fertilizers are a far better choice for cannabis.

They need some food, this is the first signs of the plant drying out and will either just be a really stunted plant or the worst it could end up dying. If you are on a budget there are a bunch of ways to feed with homemade nutrients but I'd recommend buying some if possible. Good luck and don't be discouraged just keep learning and growing! One love!

Lack of food is very too early in my mind. More likely watering regime issue or ph. Ph 6 or 7 its like distance to moon for good grower, too big of a gap. Soil 6.3 is your target. Plant dont grow in swamps, constant moist can swing pots ph too.

It’s all dry nutes in the soil. The ph of the soil is between 6-7 the microbial life adjust it as needed when watered

I don't know what problem you're asking about - but from the look of the plants, they need some nutes and I don't see any information about that in your diary so would have to assume you're not feeding them. The other issue I see in your diary is your pH - it's really too high... bring that down between 6.0 and 6.5 - but start feeding them some good NPK grow nutes! Good luck! (And thanks for the diary!!)