Leaves curling under

Growing in dirt.
What is the most common reason for leave tips curling down?

Over watering.

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

try these places first, dont forget to search the site first then maybe google. hope i helped out.

over watering or lights too close