Yellow leaves every time a plant goes into flower.

The leaves on plants going into flower are turning yellow, browing on edges, and dropping. This seems to be happening every time a plant gets a week into flower and gets worst through out the flower stage. I think the buds are sucking nutes from the leaves. What are some common causes of this?

Im growing with synthetic nutes - GH three part. Coco grow medium. 1000 ppm nutrient strength. ph 5.8

Also using CannaBoost at half strength and CO2 at 1500 ppm.

Whats the ratio? The only experience ive had is either the nute strenght isnt enough, like youre not building up its strength or keeping up with its in take ....or its root bound.

Do u ever flush b4 flower? I think coco retains alot of salts that build up, I never use coco tho