usually a juvenile or young pb is more harder to feed on growth and care
they can be stress, water difference, acclimatisation and temperature changes, tank mates intervention, space and shelter, alot of consideration
i too had kept smaller versions and failed too to realise
i guess the stable ones gotta be about 8-10 inches to thrive easily at no problems when you bring back from your loca fish shops into your aquarium or pond
could be the younger pb need to eat smaller feeder fish as they instinctly needs such food
water condition wise, i am not worried as pb in my IOUS tank has live happily in waters unchanged for 6 months , maintainence is on a low, adding weekly water to maintain levels plus rinsing of media only, so not a problem
but i believe the young need tender loving car (TLC) and waters need to be fresh as ever for them to feed better
cheers!