I dont have any themes or extensions that's not default.
Now I found that the corners are not re-sizable in all the windows.
the only customization i have done is
gsettings set org.gnome.shell.overrides button-layout "close"
Now I find that corner resizing option is available only in the bottom 2 corners of every window not on the top.
I have tried using other user accounts too. No difference.
So i dont think its related to my shell overrides customization.