What’s my issue?

I have 3 strains all about 4 weeks from harvest that are starting to slowly look like these... what am I missing?

Nute burn deluxe.

Damn I’ve been hitting them with a h2o flush every 3rd feeding hoping to prevent any burn... should I just hit them with 2 flushes back to back n then do like a 1/2 feed after? Only have 3-4 weeks left so know this part is crucial