Sorry all, I found the actual problem that makes the .elevate shadow gone missing is that :
If you have a elevated panel (Already with shadow)... and inside the panel, you have a image view (With an image), eventually this setup makes your panel elevated shadow gone missing. If i hide the image view, The shadow of the panel appears. (Panel width and height = Imageview width and height)
This is a little weird... Anyone? Need to solve this..