My budgie is not growing.

4 months back, i brought 3 baby budgies. One of them is still in the same condition, he is not growing, he don’t have much feathers and his beak has overgrown. I took him to the vet , they trimmed his beak and gave a medicine for his growth. His beak started to grow again and I trimmed his beak properly but his feathers are still the same. His height is as big as a adult budgie.Is he ok or he has some problem? Please help.