If you are serious about rectifying this recurring problem then you have to go back to basics.
1. Do they start life in 4 gallon pots with the same mix
2. Does your medium dry between watering
3. Calibrate your metre and start using it (incorrect ph or ec ranges will affect plant osmosis and limit nutrient availability)
4. What is your average ROOT temperature (if your pots are sat on a cold floor or you water is too cold this will also affect uptake of necessary nutrients and can lead to associated problems)
5. It is good practice to measure run off ph and ec levels, to make sure they are within optimum ranges
6. Have you considered using a diluted base nutrient in either your water or, as a foliar feed (never past bud set unless your temps/humidity and extraction is on point).
7. Get a bag of ditomaceous earth and sprinkle a couple of centimetre layer ontop of your medium, on every pot, for every grow. Prevention is better than cure and with that in mind,
6. Get some yellow sticky traps and hang them everywhere.
Treat it as an experiment and label a few pots, try watering less on one plant, diluted feed to another, foliar to another and so on. Use this set back to up your growing game bruva.