The things I would be leaning towards would be nutrient salt build up or a pH imbalance that has built up in the root zone, however…
We could use more info. What are the parameters in your grow?
Let’s get you started with a ILGM support ticket that Latewood had first developed. It’s is a great checklist of things in your plant’s environment that might help give us an idea of what could be going wrong.
Answer all the relevant questions the best you can and we might be able to get an idea of what is going on.
If you do not know, or do not use something, just say so or put ‘NA’.
ILGM Support Ticket:
What is the strain and type(strain name, unknown bag seed, regular seeds, feminized seeds, auto-flower, etc)?
Indoor or Outdoor? If outdoor, planted in ground or in a container?
Size of space (max height and area, length/width)?
Soil or Hydro? Type of Medium used? System type?
pH? Of the soil or medium (root zone/reservoir/runoff) and of the water and/or nutrient mix that is fed to the plant?
Type and strength of nutrients used? NPK? EC/TDS/PPM levels?
Temperature? Day vs. night temp or highest and lowest temps? Root zone temps?
Ventilation system? Size? CFM? CO2? AC, Humidifier, De-humidifier?
Number “weeks/days” from into Season, Vegetative Growth or Bloom/flowering?
Add anything else you feel would help us give you a most informed answer. Feel free to elaborate, but try to be brief and to the point. Short and to the point questions and facts will help us help you