Help please

This is about the 4th week into flower and I'm seeing this on a few of my purple kush plants. It's my first time running this strain my green crack plants the same age look fine.

1400 ppm of emerald harvest 3 part line and I add cal mag
5.7 ph
Ebb and flo with hydrotron
Hps lighting
Co2 if that makes a difference diagnosing idk lol

Are you using their feeding schedule with increased micros at this stage?

Yes I'm following their schedule in the "late flower" stage. I also add cal mg

No I'm still at mid flower on the schedule sorry couldn't remember

It's only 3 plants out of a bunch that have it bad. And there 2 different strains