i had this same issue with the same medium as you. one of my problems was too small pot. so i transplanted in 5 gallon. next is have you checked you run off ph???? i bet that is the main problem rather than the pot size but when roots tend to hit bottom, the tips curl down, ph makes the leave curl like an upside down canoe. when plants go into flowering stage the soil tends to get salty and acidic with ffof. you gotta do a light flush, check your ph runoff and raise it. check it, i bet your going to get a low ph.