Over watering and insufficient light. I agree with @Nug-bug.
If you join the forum we would be better able to help you. More information is necessary to solve your problem.
To create a grow support ticket we need some basic information to help diagnose the issue.
COPY/PASTE the below list into your forum post.
Answer these simple questions the best you can.
If you do not know, or do not use something; Just say so = NA
Strain; Type, Bag seed, or NA
Soil in pots, Hydroponic, or Coco?
PH of runoff or solution in reservoir?
What is strength of nutrient mix? EC, or TDS
Indoor or Outdoor
Light system, size?
Temps; Day, Night
Humidity; Day, Night
Ventilation system; Yes, No, Size
AC, Humidifier, De-humidifier,
Co2; Yes, No
If you can, upload a picture of your plant. It helps the diagnosis, if needed.
Add anything else you feel would help us give you a most informed answer. Feel free to elaborate, but short, to the point questions and facts will help us help you