The two BBMs in the back seem to be some days ahead.they drink a lot more now , watering 2l per 3 days.
Might have flipped to flower to soon seeing the middle branches being quite young.
Added some 2-10-2 guano for flower. 2 teaspoons per plant. Watered thoroughly after.
I don't believe your soil is depleted. Roots cant absorb the nutrients that are there when moisture levels aren't correct/ theres not enough dry out. The small root hairs begin to rot when the average moisture is too high. You might have to slightly adjust your watering amount to less, or adjust the time between waterings.. Thats my 2 cents based off what I see