I have this same problem and have been going through some processes to revive my plants. I have several strains going right now, OG platinum, kushberry, northern lights and a unknown strain, most likely a master OG mixed with something. the NL, KB and platinum have been the most sensative. About three weeks into flwering the fan leaves started to yellow and then they started to have rust spot and curling up. I flushed with floraclean and have since then raised my phosphorus and calmag amounts. Now a lot of the leaves have a purple color around the edges and the pistols have somewhat turned to amber. I am currently starting week six. Im at day 36. My Ph levels of the water going in are at 6.5 and coming out are at 6.4 so I think I am good there. I use the complete Humbolt line, using the premium two part soil( master A and B line) and have gone exactly by the schedule except for recently by bringing my ppm up to 1000 as several of my grower friends have recommended. I only feed nutes once during the week and the other feeds just RO water. If anyone knows what might be wrong, please help.