Poor little fluffies are definitely not getting enough duck nutrition from flock raiser crumbles. They need duck starter crumbles. As they are pekin fluffies they need additional niacin added to their diet.
Your ducklings can have duck starter crumbles for 8+ weeks. Add 1 tablespoon nutritional yeast or brewer's yeast to each cupfull of crumbles, urgently.
If they show any signs of gait problems, give each of them 1ml of High Dose Vit B complex [Durvet] soaked into a few crumbles daily and watch each one eat the crumbles to be sure they get the Vit B complex, until their gait returns to normal