It seems like magnesium deficiency,i don't know what water do you use for your plants,and if it is soft tap water you should consider buying calmag.
Plant needs Magnesium During Pre-Flowering and Flowering Stage
To accommodate this vigorous growth, plants will require nitrogen and extra levels of magnesium and calcium. Because magnesium is essential for cannabis to turn light into energy, not getting enough will also negatively affect bud growth and yield.