Flight feather doesn’t grow. Help please

We have 4 budgies (2 male and 2 females). One of the male budgie doesn’t grow flight feathers. The growth is uneven and he is keep plucking them and we are worried.
He is behaving normally (like food, treats, chirping, pruning and singing etc). He is not aggressive or stressed. We give them vitamins once in a week and they all follow diet plans. Mornings they are fed with vegetables and later in the day with seeds.

Please Can you help?

Thanks.