I'm thinking it has something to do with your watering schedule, and I'd actually go the opposite, I'm thinking under-watered, so you can either increase your amount
or your frequency
When I grow in a #3 nursery pot, I water probably about 1.5L every third or fourth day when small, then every other, possibly every day, when they are drinking like crazy for that early flower period, simply determined by the lift-a-pot method and how heavy they are that morning. Of course when outdoors I'm not concerned with some extra run off, I simply care about soaking the entire soil mass
Or increase your frequency of your tiny amounts of water to daily
My current indoor grow is in little 3L pots and I'm still watering about your 500mL but every day.
Happy growing and