Not feeding often enough?

Hi guys, me again!

so I’ve got these plants at day 21 of flower in 30L autopots that were being fed every 72 hours and I’ve started getting yellow leaves with green inner veins.

They have very slight tip burn and the ec is 1.8 in the res so I’m not under feeding

should I switch to a smaller 50% feeding every 48 hours? Or maybe even less water at 24 hours?

It looks root bound
But that is just a wild guess
How long has it been in that pot?

from the start.
I don’t think they’re rootbound because even some of the smaller plants are showing the same symptoms, the yellow leaves with green veins.
This photo is just before their watering/feeding so it hasn’t had anything for 72 hours, hence a bit droopy

Bump