Howdy fly9, It could be caused by a whole bunch of problems from improper pH, nutrient deficiency, critters, or LED's too close. I am not a soil grower, so I can not give You good advice for what is going on with Your plants, but if You can post all of the details of Your situation (type of medium, lights, pH of nutrient and runoff) others, more experienced, can more readily help You.
Good luck with Your grow.