we need more info to help you out such as lights, distance of lights, soil, nutes, age of plant etc. If I had to guess I'd say give the plant some calmag, or epsom salts will do the trick. give it a full dose and then back down to half dosage for a while every other watering. this should fix the problems you are describing
Sorry ha I knew I was missing sonething... 400w HPS 24"away GH nutes I added calmag a day ago so ill see off it helps