If you are trying to use one of those security lights 150w HPS it could be a hot spot is heating up the inside of the plant so monitor the heat at the top of your plants under the light so you know what temperature they are getting. If you are using rock wool with grow rocks try removing the rock wool it could be holding to much water and nutrients. Grow rocks and rock wool each requires different watering schedules. Some may never have trouble but the other 90% of us have had problems. It sounds like it may be root rot due to dampness around the root ball. This is just a hunch with out pictures.

Peace