very nice looking plants- i grew nirvanas WW in my first grow and got one in current too- its the healthiest looking one lol seems they are resilient- as i am too having a few diff probs with other strains- think they are just greedy little feckers lol

i would guess sl nitrogen def with that plant- having same prob with my Pineapple chunks so gave em some compost tea- but yours are well into flowering so a bit of def is normal i reckon, what you feeding them? + rep and gl
Agent xx