Is My Budgie Not Growing? Or Sick?

Hey guys 

I have a baby budgie about 25 days old. He/she is too small for its age. The other babies younger to it are much bigger (double in size). But this one is too small. He/she is of the same size as it was a week ago. But feathers and fur grows quite well. Mother feeds them daily. He/she is not so active, either sleeping or just looking around. I tried hand feeding it 3 times. It ate very little food. 

Also, I think he/she is not able to stand properly. He/she leans towards one side and takes heavy breaths 

Can you guys  please let me know if anything is wrong? 

Uploading a video taken today April 25, 2020. This video is taken just after hand feeding him