Why doesn't it work when height is fixed?

In the website I am making, #div-center is fixed height. I need this plugin to work especially on this page –> http://fr.annevaner.be/livre-dor/
When clicking on “Ecrire un commentaire” (Leave a comment), I’d like that it scrolls smoothly to the comment form which is at the bottom.

On the other pages, it’s .post-bodycopy which is fixed height. I’d like the plugin to work in that case too.

Is there a way to make that work ?

http://ww.wp.xz.cn/plugins/easy-smooth-scroll-links/